[morley]

I don't read "it exists because it has to exist" in the parent's statement. They're saying that there's an advantage one way and a disadvantage another, and evolution favors advantages. I wouldn't characterize a statement like that as a tautology, and I don't think the author deserves your dig for it.

[ars]

> and evolution favors advantages

The question was how, not why.

Your answer is like saying "how does the eye focus light" and answering "so that you can see".

[VeninVidiaVicii]

In my opinion there’s a general fundamental misunderstanding on the purpose of theories. I see it all the time — attempts to explain why something is useful simply because it exists (re: popular science evolution). There are loads of suboptimal traits that are counterbalanced by something else.