This is important because people often treat evolution (and causal explanations in general) as an argument for moral skepticism: whatever caused our beliefs, it presumably isn't the abstract moral facts themselves!* But such arguments are question-begging, for they presuppose the skeptical view that our moral beliefs aren't justified.** Indeed, I think there's an important sense in which our philosophical beliefs are caused by the facts: we are responsive to considerations of rational coherence, which is precisely what the truth itself consists in.
* = It's also suspicious that only moral philosophy is singled out here. Logic is no less abstract, after all. Not to mention the belief in skepticism itself.
** = It works better as an argument against Platonism, though.
Here's the vital point: if philosophical truth just is what's maximally reasonable, then the skeptic needs to show that no moral views are more reasonable than their competition (for this would suffice to explain our knowing them). But of course merely pointing to Darwin does no such thing.