what is testosterone used for if you're a male?!

Testosterone is a hormone found in large quantities in males. It works with muscle growth, sexual reproduction, and most male-puberty related bodily functions. Men specifically use it for the production of sperm, the growth of muscles, and many other "manly features". Basically anything that makes a man a man (facial hair, sperm, deep voice, muscular figure) is controlled by testosterone.

http://health.msn.com/health-topics/sexual-health/mens-sexual-health/articlepage.aspx?cp-documentid=100200703

As a male you naturally have more testosterone in your body than estrogen, and as has been previously stated it makes "male" things happen in your body.

Unless it is under a doctor's care and watchful eye, it is not a good idea to take much extra testosterone as this can unbalance the chemical make-up in your blood and often have bad side-effects.