Amino acids in muscle developement
Taken in through a normal human diet, amino acids play a critical role in muscle development - basically, they are what muscles are made of! There are a total of 20 different amino acids, 9 being essential for the body (we cannot make these on our own).
You will find amino acids in any food product that is a source of protein, however they are found in highest concentrations in foods such as red meat, diary products, poultry, fish and other seafood. Vegetarians will get their sources mostly from legumes, peas and grains.
Usually individuals who choose to supplement with these include those who are vegetarians (since there is a decreased chance they will get enough in their diet), people who suffer from allergies, those who are experiencing stress-related fatigue or those with hypoglycemia.
Some coaches and athletes feel that high dosages of certain forms can lead to an increased athletic performance and while there is research to back this up, most important is just ensuring you are getting a well balanced diet. If you take one athlete who is not getting enough total protein in their day and boost that intake up, that in itself is going to make the most significant effect - much more than if you had just added one specific supplement.
A good target to aim for is approximately 0.8-1 gram of protein per pound of body weight per day. Those who are doing a lower calorie diet may want to bump this up to 1.5 grams per pound to avoid muscle loss however intakes above that are simply not necessary.