Maybe for other elections or offices they serve a function, but these are stage managed farces. Anything you want to know about a candidate you can find online, they aren't going to truly answer any serious questions, just use the question as an invitation to parrot the closest talking point their advisers had previously drilled into them. The performance in a debate doesn't prove anything, and such performance is absurdly subjective anyway, as proven by the infamous discrepancies in the perceived winner of the Nixon-Kennedy debate between the audiences in different media.

I don't know what a debate is supposed to prove about one's capacity for the office anyway. The president has people to do that for him.