A Meta analysis of all current data was undertaken to see the effects protein has on muscle mass gained and strength when weight training. The results came back to show that the supplementation does have an effect but not a very big one!
So the very simple answer is yes, protein supplementation helps you gain muscle when you lift weights, but it is not much. Protein supplementation also helps you gain strength, but again it is a small effect. Importantly, for reasons we do not understand, protein works better in trained persons. If you are over age 60, then extra protein still may help but it is an even smaller effect. Finally, a daily protein intake above a level of 1.6 g/kg (or 0.7 g/lb) does not appear to help you gain more muscle with resistance training.
This leads to the question should we all be spending so much on supplementation to aid sporting performance or can a stable diet and variety be enough for us? or does this show that once you become a trained athlete that supplementation can give you the added edge that you need, that extra 1% you may need in performance.