Many theories but no good results to explain why it exists:-http://www.the-scientist.com//?articles.view/articleNo/40333/title/The-Sex-Paradox/-"In a static world, sex is likely unnecessary. The ever-changing environments of Earth call for a different scenario, however. Sex allows species to adapt to the loss of food sources, the arrival of parasites, rising temperatures, and more.
 
"There is some doubt, however, whether the environment fluctuates fast enough to warrant the prevalence and persistence of sex in the eukaryotic kingdom. “Is the force favoring sex large enough in the face of the costs?” asks Otto. “The niggling doubt in the back of my head is that it is not.”
 
"And so the paradox of sex lives on. “We still really don't know the answer to this very most basic question,” says Mark Welch. “We don't know why sex exists.'"