Protein is a chain of amino acids and provides the building blocks for all the cells in the body. It is particularly important for muscular growth and repair, and can be used as a fuel source.
It is recommended that people eat 0.8g of protein per Kg of body weight. This is enough to maintain current body mass and is sufficient in providing for any possible energy needs. However, when resistance training is undertaken for muscular hypertrophy protein consumption should increase to 1.2-1.8g per Kg of body weight, in order to grow muscle size. It should be noted though, that this upper limit for an athlete weighing 100Kg is still only 180g, which would easily be consumed in a normal western diet, because meat, dairy and eggs are high in amino acids.
Supplementation is not required for most western athletes. If the athlete is a vegetarian or vegan amino acids are usually consumed through beans, nuts and legumes and would need to be monitored in order to ensure adequate amounts are being eaten. If not, then supplementation may be beneficial.
Supplementation may also be beneficial because it can improve the timing of protein consumption. Protein consumed within 1hr of resistance training may help recovery and increase hypertrophy. However, excessive consumption can have a detrimental affect on health, particularly kidney function.