It also changes the fly's appearance to the fish. The hackle is meant to look like legs or wings, or at least the pulsating movement of an insect or other aquatic life swimming. At the very least, the movement of the hackle in the water makes the fly look like something that's alive. Too much of it probably doesn't look quite correct to the trout. Bugs don't have that many legs anyway.
To tie up an effective soft hackle wet fly, all it takes are a few (2 to 3 at most) turns of the feather around the hook. Of course, the type of feather that you use is very important. Not just it's coloring, but maybe the most important is it's softness. There's a reason the old recipes call for feathers like partridge or guinea hen. It's because of not only their colors, but because these soft feathers move & pulse in the water, just like a living creature.
Another concern about hackle on wet flies is the length of the feather. Some feel that the feather shouldn't be any longer than the legs on the insects in the water where you'll be using the fly. Others like to see their flies with very long hackles that extend well beyond the bend of the hook. It seems that the jury is still out on this one. Both styles of hackle tying seem to be very effective.
So as you sit at the bench to tie up some flies for the upcoming fishing season, don't forget about those old, reliable soft hackle wet flies. They're very easy to tie & extremely effective on trout - especially in the early part of the season. As you tie up some of these wets, just remember to not go overboard with too many turns of the hackle.