Weight gain supplements aim to give you a mixture of protein and carbohydrates to fuel the body in its pursuit of building muscle mass. Complex carbohydrates restore muscle glycogen that is lost during intense exercise and protein aids the body to build new muscle. Taken immediately after exercise carbohydrates and protein work to stop muscle breakdown. Eating these key nutrients directly after your workout has more benefit that eating the same nutrients an hour later. Taking powdered weight gainer to the gym is a convenient and cheap way to consume these essential nutrients.