Decreases STRESS!! Stress exacerbates (or causes) nearly all medical conditions. Stress decreases immunity, leads to heart disease, autoimmune conditions and cancer.  The entire body runs much better without stress.  Massage therapy not only decreases stress once it's present, but also helps prevent stress from developing in the first place by causing the reduction of physical stress (e.g. back pain, shoulder problems, etc.)
Increases circulation There is a very dramatic increase in circulation when a massage is given. There is also a temporary rise in blood pressure during a massage, followed by a decrease in blood pressure. Massage does great things for the cardiovascular system.
Relaxes soft tissues The soft tissues of the body (muscle, fascia & other connective tissue) can put pressure on nerves and not allow the nervous system to regulate body functions properly.

Best time to get a massage Massage Therapy is most effective when you're feeling perfectly healthy. Your body has a memory. If you allow your body to be in a dysfunctional position for a long period of time, it will go back to that position even after the problem has been resolved. People have a tendency, if they're feeling a little achy but not in a lot of pain, to feel like "it's not that bad, I don't want to spend the money". But as their dysfunction increases it becomes "it's really bad now, it'll be worth the money". But actually just the opposite is true. I had a client once who would only come in when his back was absolutely killing him. By the time he came in he was in such bad shape that he'd need to come in three times a week for two or three weeks to get him back in shape. I told him that if he'd come in when he's feeling healthy he'd only need ten or twelve appointment per year to keep him that way. He'd save money, he wouldn't get the back pain and he'd enjoy the massages more.
Overall, Massage Therapy helps improve health, beauty and fitness.