Ladies are traditionally known for being more relational than men, so they sometimes can see dinner conversation as being as important as a good TV show, and look for unimportant "breaks" where they can cut out and pee. If one lady decides to go, another lady might notice it, think about where the conversation is at, and realize, "Oh yeah, this probably is a good break." By the time you get to lady No. 3 or 4, she's not only thinking that, but, also that with a bunch of ladies gone, there might be a lull in conversation -- or at least lady-appropriate conversation -- and this is as close to an official break as you're going to get.
When it comes to ladies on double dates or in large prom groups, they might see it as a good time to catch up with each other and how they think their dates are going. Maybe say what they think of each other's dates. Just like men enjoy the play-by-play in sports, ladies enjoy a play-by-play on dates, but it's very rude to do it right in front of the guy.