While I feel it is always dangerous to speculate why animals do what; I have read that eyes on a critter indicate the head area and observation as well as the potential for danger, (teeth) so predatory fish will attack from the rear.

So with that assumption in mind; an eye would add a realistic element to a fly of substance so for that reason I think that it can add to its effectiveness.

Besides; flies look cooler with eyes and coolness is about the only reason I need to add an element to my flies.

I don't get hung up on size or color but I'm partial to a yellow, white and red iris in that order although I once made a "Drunk Deceiver" that had bloodshot eyes. I also mostly use dumbbell eyes or Eye Balz, not flat doll eyes. Durability is one thing I strive for in a fly and doll eyes don't seem to stay put as well as dumbbell eyes for me.