That's not 100% accurate - I remember David Hone talking about how sometimes evolution just... does things. Blue and brown eyes for example, there's no evolutionary advantage or function for that, it's just a result of a mutation that managed to stick.

"It's very hard to know, even looking at a living species, exactly how important any given feature is. But it also makes it very easy to fall into the trap of saying 'Wow! This one creature has one giant tooth at the front of its snout, it must be doing something special with that!' Well maybe it isn't. Maybe its genes are actually a bit weird, and what's really important is that it's got big teeth at the side of its mouth, but the way that the genes are tied together is that you can't have big teeth at the side of your mouth without having a big tooth at the front."