Are the roles specified in a Founders Agreement (contract) legally binding?
Yes, like pretty much everything that is provided in a contract. Information about roles is useful for ascertaining the rights and duties to which parties/partners knowingly and willfully agreed.
Does having the role of the CEO assigned to you in the contract, for example, give you certain legally binding responsibilities/limitations, or are these roles just symbolic?
It is too risky to assume that something in a contract is symbolic. An assumption of that sort is indicative that one might not fully understand what the contract entails (and consequently the vulnerabilities inherent thereto).
Although some labels and acronyms of roles are self-explanatory as to hierarchy and scope, it is in the parties' best interest to ensure that the contract is detailed enough to preempt conflicting interpretations (ambiguities). For instance, the Technology and Information departments are likely to overlap (the extent to which that happens depends on the type of entity being founded), whence it makes sense to include sufficient information in the contract so that a reasonable person can ascertain from the Founders Agreement what issues correspond to a CTO and which ones to a CIO.
it's in fact mandatory in a template I'm using.
Besides unlikely, it is unclear whether statutory law in your jurisdiction requires you to use that template. Templates are inevitably generic and might be inadequate for reflecting the parties' intent. Trying to fit a template is likely to distract the parties from the intent they would otherwise memorialize with the clarity and specificity they need.