It might look like there are separate male and female things unless you look closely, because not many women play. It's easy to miss them. For instance, on the top 100 list, there is only one woman, Judit Polgár, down around #50. She has been as high as #8.

I believe that the usual argument for women-only tournaments is the same as the argument for women-only hackathons or women-only coding boot camps. Like programming, chess is a de facto male activity, and so women can feel uncomfortable participating. A woman-only tournament gives them a chance to play without hang to deal with being part of a tiny minority.