As mentioned above, eyes have evolved separately many times. It's good to see.
Echolocation evolved separately in the sea and in (e.g.) bats. It's good to hear.
Flight has evolved in mammals, birds, and fish. It's good to fly.
Color vision evolved in insects and fish. Plants evolved brilliant colors to advertise to insects. It is thought that very few mammals have full color vision: then why does Man prove to be the exception?
Turtles have far better color vision than man. Why? I don't know. I can tell you how, but not why. The question of "why" begs the question of teleology, a doctrine explaining phenomena by their ends or purposes, and some evolutionists make the mistake of asking teleological questions. For their error the rest of us life scientists pay the price of having to defend the undefensible.
Until we find hard evidence of the creator and the blueprints, we will never know "why."
In the meantime, science muddles along trying to explain "how."