The purpose of vinyasa is internal cleansing. Breathing and moving together boils the blood. Thick blood is dirty and that causes disease in the body.
Heat created from yoga cleans the blood and makes it thin, so that it may circulate freely.
Combination of the asanas with movement and breath make the blood circulate freely around all the joints, taking away body pains.
When there is a lack of circulation, pain occurs. The heated blood also moves through all the internal organs removing impurities and disease, which are brought out of the body by the sweat that occurs during practice.
Sweat is a crucial by product of vinyasa, because it is only through sweat that disease leaves the body and purification occurs.