I always wondered this too....and I did a bit of research on it. The thing is that Western culture has very little understanding of some of these ancient customs...and much of the truth has been bent and distorted by time and ignorance....older cultures believed that women were ruled by the Moon (and even now in the Western world many women's periods are timed by the Moons cycles) ....these cultures also believed that when a woman was on her period, she was very powerful....this is partly because they did not fully understand how a woman could bleed that much without dying and partly because they believed the Moon to be magical and so the woman would have some of that power within her. So, when her period arrived, the woman would go to a separate tent or hut....so as not to ruin the days hunting....and so she could be peaceful and alone at a time of physical pain or discomfort...our modern society has twisted this into women being segregated because they are dirty when they bleed.