For example, if the evidence is overwhelmingly against them and the defendant knows they are guilty and there is no chance for a lesser sentence or a plea deal (happens in certain jurisdictions), is there still any merit to continuing with a trial?
Juries aren't terribly accurate. There is an irreducible chance that no matter how clear the outcome should be that the jury will get it wrong. Based upon a review of the academic literature on wrongful convictions and inaccurate acquittals, I generally tell my clients that this is about 10%. Many people think that this is a low end estimate.
Also, sometimes a jury will acquit a defendant in a case where they think that the defendant was actually legally guilty because of extraordinary circumstances, and so the jury will disregard the law and acquit. This practice is called "jury nullification."
And, as other answers have noted, sometimes the prosecution or the judicial system screws up for reasons that are unforeseeable, after a not guilty plea, in a way that makes proving your guilt difficult or impossible.
Basically, if you "roll the dice" there is some non-zero chance you will be acquitted, while if you plead guilty, there is none.
Also, sometimes court decisions will change the law in way favorable to you after the trial, and as long as your case is still on direct appeal from the conviction, you can benefit from those changes in the law, which you cannot if you simply plead guilty without any concessions.
Likewise, if you are innocent and the evidence is currently strongly against you, but you wish to preserve the ability to later attack the conviction based upon future newly discovered evidence, not pleading guilty is generally necessary to preserve that option.
Another circumstance where going to trial but losing can still be worth it, is where there are extenuating circumstances that make your conduct understandable, even if it is not a legally valid defense. Getting these facts in front of the judge in a fuller fashion, as a trial can make possible, can convince the judge that while you are legally guilty, that you deserve leniency.
Going to trial typically results in a longer sentence, even without a plea bargain, however, so going forward with a hopeless trial is rarely a good move.
The defendant cannot be sure that the evidence will leave the trier of fact (judge or jury) with no reasonable doubt that the defendant committed the crime. Thus, the defendant can only "know" they are guilty in the colloquial sense (in that they did the things that could be found to be a crime). However, they cannot know that they will be guilty in law. There is always a chance that the trier of fact is not convinced.
Ways that the prosecution's case can fall apart at trial:
- what appeared to be convincing to the parties just isn't convincing to the trier of fact
- a key witness or evidence becomes unavailable
- it turns out critical evidence was obtained contrary to constitutional principles
- a key witness's credibility or reliability crumbles on cross examination
- the prosecution is complacent in diligently moving the trial forward and the trial drags on to the point that a stay of proceedings is warranted (e.g. in jurisdictions with strict rights to timely trials)
A person might simply desire to exercise their right to force the prosecution to prove their case. This can have important benefits to the accused's sense of dignity.
In the English legal system you get a small bonus for pleading guilty early. Since that might equate (in a murder trial) to less than a few months of additional jail time after a potential sentence of decades, it's often advantageous to go to trial and hope to win, despite the long odd of doing so, rather than pleading guilty.
Anecdote time. I saw a case in court a few months ago where the defendant had admitted during interview that he had stabbed someone in a bar fight, with the fight itself being captured on CCTV. The prosecution lost the interview recording in their computer system (along with the footage) and were relying on the interviewing officer and the victim to testify. When they failed to turn up, the CPS asked for a stay while their IT guys recovered the footage. As there had already been four delays the judge ended the trial and acquitted the defendant. The guiltiest man in history walked out of there a free man.