Historically, women did not feel welcome at open chess tournaments. Chess talent was seen as synonymous with intellectual brilliance, so some men would not take it well when they were beat by a woman. I like to think that this has changed, but there's probably still an element of this. Regardless, women's tournaments were set up so that women who wanted to play chess competitively would have an opportunity to do so safely and without the fear of harassment. They still exist because people still compete in them and enjoy them, so why stop?