protein simply put is the building blocks for your muscles. when you work out you break down your muscles, and protein is used to rebuild them, in the case of extremely strenuas excersize, bigger and stronger. why protein is an easily digestable form of protein in a large quantity, when you finish working out your muscles crave protein for repair, not feeding them results in little gain in size or strength.

the problem with alot of protein drinks are they contain alot of calories, something a boxer watching his weight doesnt need. if your in the middle/bottom of your weight class and need to get a little heavier to be at the top, then use protein supplements (as long as your of the age and have done your research.
people looking to maintain probabely shouldnt use them (unless they big guysm in which you need protein to keep your muscles from wasting)

people looking to lose are a difficult subject.
protein shakes can be used as great meal replacement, they stimulate your metabloism and stop muscle catabolism, however can put on extra weight/muscle if taken aswell as your daily eating.