Because they feel it will give them bigger gains.Testosterone boosters are marketed to men to build muscles.While baseline levels are varied for men between 300-1200 most men feel that if they take a booster that will send their levels into the 1000s it will give them bigger gains.
While someone whose baseline is 300 may think it will work for them to get their level to 1200 it will do absolutely nothing for them.In order to make gains one must LIFT HARD & EAT RIGHT!