When you have tight and constricted muscles, the everyday buildup of metabolic waste is not flushed properly due to poor circulation in those areas. Massage can release these tightened muscles allowing for the waste to be flushed out. This is why it is so important to drink water after the massage, because lack of extra fluids can cause the waste to stay put and the muscles to tighten up again.
Drinking enough water on a daily basis is very important for the overall functioning of the entire body. It can keep your muscles from becoming overly constricted and allow for overall improved circulation.