Muscles grow from training by breaking down and then rebuilding stronger than before to prevent future breakdown, basically. Hence you need enough protein to repair the damage and a bit extra for the reinforcement.
Train without enough protein and you get the breaking down bit without the protein to repair, ie you can actually lose lean muscle tissue.
Some proteins the body can manufacture itself from other food sources but not the 'essential' amino acids, which must be obtained directly from your diet.
B.