What's with the pocket in women's underwear?

Asking because I'm new to wearing women's underwear and realized in all my years I'd never questioned why the double stitching in the inside crotch of women's underwear has one side open. Almost like a little pocket for nothing.

I know why there is an extra layer of fabric. It's the fact that it's sewn into the rest of the underwear on three but not all four sides I find curious.

Genuine question, does anyone know why?