Testosterone (an androgen) plays an important role in male sexual development, and in sexual arousal and response (although its role is nuanced). Androgens, including testosterone, stimulate muscle development, which is why anabolic steroids, which are androgens, are so popular with bodybuilders. By injecting large doses of anabolic steroids, bodybuilders can put on much more muscle than they could naturally.
Androgens, in large concentrations, can have a freakish effect on muscle development (along with lots of hard work in the gym):
And not to be outdone by the men: