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In a federal state the federal legislature and the legislatures of the regions within the federation each have their own particular roles and competence which is defined in the federal constitution.
In a unitary state there is only one legislature or, at least, there is a single legislature which is supreme and the roles and competences of any other legislatures can be changed or abolished by the supreme legislature. So if there is a supreme legislature what is the point of a constitution?
The primary purpose of a constitution is to define the structure, operation and limits on government. It contains the laws that the government cannot change except by any procedure included in the constitution.
It isn’t necessarily to spell out the rights of citizens. Some do, some don’t.
The median age of constitutions is 19 years - half of them are older than this, half of them are younger. Coincidentally, or perhaps not, this is close to the 20 years that Thomas Jefferson thought a constitution should last since “the earth belongs to the living, and not to the dead.” The US Constitution is the oldest (and shortest) still in modern use - it’s influential but it’s a poor example of what modern constitution’s are like.
For some particular examples:
As you say, anything not specifically forbidden is presumed to be allowed. But the legislature can (relatively) easily pass laws to forbid things. Most constitutions make it significantly harder to amend themselves. So enumerating specific rights in the constitution ensures that they will not be taken away without a real struggle.
For example, the 2nd Amendment makes it impossible to pass laws forbidding gun ownership, and even makes it hard to pass laws regulating the process of acquiring guns.
There's also a philosophical aspect to this. A nation's constitution describes the fundational principles of the nation. Enshrining certain rights in the constution indicates that they are of supreme importance. These aren't just laws, they're what the nation represents.
Consider, for example, how the US 2nd Amendment empowers gun-rights advocates. Or how often people invoke the 1st Amendment's rights to freedom of speech, religion, and the press. Denying these rights is considered "un-American".
On the other hand, the Affordable Care Act (AKA "Obamacare") is just a set of laws. Many politicians and laypeople disagree with the principles that it espouses, but would not be considered un-American on this basis.
A constitutional law is harder to reverse than a simple law
For example, France absolished the death penalty though a simple law, which could have been reversed through another simple law, with 50% of the lower house.
It was added to the Constitution in 2007 (Article 66-1: "No one shall be sentenced to death"), and now requires a two-thirds majority in both houses: much, much harder to achieve.
Constitutions are more than their Bill of Rights, and in fact, the U.S. Constitution was first written without the Bill of Rights (They were added in through the amendment process shortly after the document was ratified).
Constitutions are the supreme law of a nation and will outline the nation's government structure (Who is the Commander in Chief of the military? Who writes the Laws? Who is head of government? Who is head of State? How are the courts set up? Who can override who and when can they do it and how?) as well as delegate authorities to different government branches.
Consider the First Amendment of the U.S. constitution, which begins with "Congress shall make no law...". This essentially establishes that it's beyond congress's ability to make a law that punishes speech or religious expression, thus a limit on the Federal Legislature's powers. The States were not so bound until the 14th Amendment was passed, which among other things, forced the states to adopt the Bill of Rights. Similarly when amendments to the constitution banned things, (13th which banned slavery and 18 which banned alcohol) both Amendments included language to give Congress the power to make laws to enforce those bans (since those laws were previously powers of the state). Those laws actually gave the amendments some teeth to enforce the bans. The 21st Amendment (Which banned the 18th Amendment didn't include such language as repealing Prohibition reverted the powers to regulate alcohol to the states again.). A specific amendment (10th) actually explicitly makes it so the Federal Government may not assume power which the Constitution did not delegate to it by making it a right of the state or the people.
In fact, even in Federal Nations, Constitutions still delegate soveriegn powers to the member states and each member state has their own Constitution (The California Constitution is one of the longest such documents in the world, even though the United State's is one of the shortest in the world). All U.S. States are themselves Unitary States with devolved power to non-sovereign sub-regions (counties, Parishes in Louisiana, Burroughs in Alaska) that are created by the state's constitution.
Unless something is explicitly forbidden in law it is allowed, so why do unitary states have constitutional rights? There is no level below them that could mean differently. Nothing would counteract a constitutional right if it was removed.
Any state with a constitution of any form has rights that arise from it. Any kind of law establishes rights and responsibilities, and constitutions are not an exception to that rule.
In Europe and other places with a constitutional tradition of parliamentary sovereignty largely unfettered by judicial rule, the primary mechanism by which human rights are protected that is resistant to ephemeral or bare majority parliamentary action, and which is enforced by courts, is through international human rights treaties. In Europe, these were devised and administered largely by an international organization parallel to the European Union called the Council of Europe which promulgated and secured widespread adoption of with little qualification, the European Human Rights Convention.
Most Western democracies, unlike the United States, provide that a conflict between a duly adopted treaty that remains in force, and subsequently adopted domestic legislation, is resolved by the courts in favor of the treaty. These treaties are the strongest protections of human rights in most of these countries that are enforced in their own domestic courts and regional international human rights courts which countries in such treaties take more seriously than the U.S. does.
There is circumstantial evidence from the structure of the U.S. Constitution that the drafters of the 1789 Constitution of the United States intended the same thing, but the Courts promptly settled on the opposite rule flowing from a very literalist reading of Article III of the U.S. Constitution that doesn't consider intent or context and now has the force of precedent behind it.
While it is much easier to amend constitutions in most unitary states (which are frequently amended much like U.S. state constitutions), many still do have human rights protections in their constitutions, largely as a result of a movement when constitutions were redrafted following World War II which interrupted most of Europe's national regimes at least temporarily due to Nazi occupation or rule, on the model of the United States Constitution and as a way to honor their new membership in the United Nations by incorporating much of the U.N. Declaration of Human Rights (a non-self-executing instrument) into enforceable domestic law, usually supervised by a separate constitutional court. As recent shifts away from a consensus around those rights have eroded, treaty rights have proven more important than constitutional rights (e.g. in Poland and Hungary). But inertia, and the popularity of the general idea of protecting constitutional rights even though particular aspects of those rights may be unpopular, has kept those protections on the books as a political matter even though they wouldn't be hard to repeal from a process standpoint.
A constitution does not grant rights the citizens already have. It prevents the state from taking those rights away.
For example, when a constitution says "Every citizen has the right to mow their lawn on Sunday morning", then that says that the legislature is prohibited from passing a law which infringes on that right and that the executive is prohibited from enforcing laws if doing so would infringe on that right.
One point that hasn't been detailed in other answers is how this affects the balance between different institutions. In practice, a written constitution with a bill of rights often empowers the courts to nullify actions by other branches of the government. International human rights treaties can play the same role, even in places where court activism is traditionally frowned upon like France.
