Can someone explain to me why does girls go to the bathroom in partners or groups?

That's something I'll never understand. I would never want a guy friend to follow me into the bathroom. I remember on a road trip my friend went into the bathroom at a rest stop and though there were other stalls, I waited for him to leave before I went in. It's just awkward and I wouldn't have wanted to talk to him while "going". Girls are comfortable with that kind of thing, I guess.