LOTS of variables contribute to the final product:
kind of smoker, water/sand pan or magic saucer in pan (if wsm), the actual meat itself, how full the smoker is, temp spikes, the rub, amount thereof, basting/spritzing/leavin' it the hell alone, flipping/rotation/leavin' it the hell alone, dry/drowned wood, age of wood, kind of wood, amount of wood, method of using the wood, placement of wood, charcoal used, appetite of cook, mood of cook, sobriety of cook, time meat rested, temp rested at, time cook rested......:-D Blah, blah, blah
With all that, it's hard to definitively say that foiling helps in EVERY situation, but it might work for YOU.
It definately makes the meat cook faster and more predictably, and obviously protects the bark from excessive smoke, so I'm not surprised it's used by competitors. It's also a way to add some a little more flavor, and yes, you can always open the foil to firm up the bark on the final product. Blah, blah, blah...sorry
Funny when you think about it, but it's probably the best route for the novice and pro competitor alike. However, I usually get just as good a product on my wsm by not foiling until resting. The wsm water pan makes for a very moist environment (think softer/less bark) and I cook around 250*...butts on the bottom (more forgiving) and brisket on the top grate.