Here are my reasons:
It’s very unnerving to walk across a room, especially if it’s at a party/club/restaurant and have strange men stare you up and down, even more so if you are wearing a dress or heels, which unfortunately has happened enough times for me to become uncomfortable with it. Having another person with you makes this situation less awkward, and even less likely to happen (in my experience) because some men seem to think that a woman walking alone without her date/husband or female friend, is on the prowl, which makes no sense, but it happens more often than us women would care to experience. If I’m going to the restroom, my date is not likely going down the same hallway, so it’s better to go with another female. Safety (perceived or real) in numbers.
If we’re all out at a social gathering, it’s very likely that we’re either with already with a date or we are trying to make some kind of social contact with a male that we would like to date and we don’t want to look stupid/desperate/slutty/ugly. We like to discuss everything with a female who understands what we’re going through. We will probably ask each other if we look presentable, if there is paper stuck to our shoe, lipstick on our teeth, spinach in our teeth, or to make sure our dress is not tucked into our underwear.
Many women’s restrooms are neither clean, nor convenient. I can’t recall the number of times I’ve gone into a restroom, even at nice establishments, where the door doesn’t close completely, doesn’t have a lock that functions properly, doesn’t has an adequate supply of toilet paper in the stall, or a place to hang your coat and purse. Your friend can help you with all of these things, and you can, in turn, help her.
If you’re having a good time, we like to talk about that. If we’re having a bad time, we need someone to lean on, or possibly someone to give us some advice on how to proceed from this point forward, whether things are going good or bad. It’s nice to have a sounding board.
And one of my favorite scenarios. This tends to happen more often in a club/concert setting more so than at a restaurant. You go to this event with a date. Sometime during this event, another female starts flirting aggressively with your date, or makes some comments that allude the idea that this other woman and your date have had a previous relationship, are currently having a relationship that you were unaware of, or seem to be creating a brand new relationship right in front of your eyes. You take a girlfriend to the restroom with you, so that you can ask her if she was perceiving the situation the same way you were, and if so, does she have any practical advice on how to get out of there without causing a scene, or to be seen crying in front of everybody. If she’s a good friend, she will help you clean up your mascara if you’ve been crying, she can help block you on your way out the door you need to leave immediately without your date, and she can retrieve your wrap/phone/lipstick off of the table for you, so that you don’t have to go back and get it while the other woman is rubbing up against your date.