Why do sportsmen and sportwomen put water in their mouth and spit it out?

In many sports that involve a lot of running such as soccer, AFL (Aussie Rules), Rugby League, Rugby Union etc you regularly see them squirt water or gatorade type drink in their mouth and do not swallow it. You very rarely see them actually swallow the drink. Apart from being totally disgusting, what is the benefit of spitting it as apposed to swallowing? I always thought that drinking water is what you are meant to do to rehydrate when you sweat a lot.