Agreed with 42BBQ, its best to keep it simple and eliminate unnecessary variables until you get the hang of it. With limited variables, its easier to identify where mistakes are made.
To answer your question, I find foil helpful in two areas. Once the bark is where I want it, I might foil to keep from getting excess bark or charring. This is usually if the cook is taking a while. Thus the second advantage of foil is that it helps get the temp of the meat up while retaining fluids and moisture.