Because on the off-chance that the stall you choose is out of toilet paper, you can just ask one of your girlfriends to pass some over from her stall.

Also, in a party or club situation, you really shouldn't wander off anywhere by yourself. I did that once (headed off to the bathroom alone) and I had to pepper spray a guy who latched onto my wrist and wouldn't let go.

In a double date situation, we go to the bathroom to gossip about the date.