Blood flow is key to the health of any cell. When a muscle along with the surrounding tissue is lubricated and loose, the fibers are primed for flexibility and strength training. When a joint is relieved of muscle tension it can stay properly aligned. These are the main physical benefits of massage therapy.
If someone has an injury or disease that increases blood clots, a systemic swelling condition like rheumatoid arthritis, or arterial plaque build-up; the extra blood flow does much more harm than good so massage should be avoided in situations like those.