It's the all about "me" attitude. If she thinks the guy is hot or he is famous, rich, or powerful then she wants to be (a part of his life) if only for one night. Very few of these women believe they will "steal" the husband away from the wife. Many see being "the other woman" as having all the perks without having to deal with the mundane day to day crap in marriages (laundry, kids, bills, emotional ups/downs). When she's with the married man it's about experiencing a "romantic escape" . Some people say the other woman must have low self-esteem but I believe there are some who are just in it for the fun. I personally know of one man who left his wife to be with the mistress and when he told her they could now be together openly she dumped him! Apparently she was happy with seeing him on a part-time basis and did not want the "full-time" job of being his mate.
Some mistresses of the super wealthy enjoy trips around the world, expensive gifts, having their bills paid, or having a condo/home provided. There are some instances where the husband has left money in his will to his mistress. Katharine Hepburn was Spencer Tracy's mistress for over 20 years! He also had other mistresses prior to her as well. The rich and famous men don't have to remove their wedding rings or lie about their marital status to get women to have sex with them or "date" them. These women come willingly and knowing up front how things are going to be. Some mistresses say since they haven't taken any "vows" then they aren't cheating or really doing anything wrong. They feel it's the man's responsibility to worry about his wife and family. Those things are none of "the other woman's" business. Naturally the wife disagrees!