It's nothing more than pre-selection bro. If a girl sees you with another girl, jealousy ensues and makes other girls think "What does she see in him that I don't?" Or "Why the f**k is she hanging out with him?" Or "What the f**k did he have to offer to her?" I must know, he's got to have something interesting about him or he wouldn't have a lot of value. And the fact that a guy's value is raised quickly and easily by having a woman with you is something that makes you more desirable in a woman's eyes. They say the best way to get a girl is to get a girl. That explains some guys going to clubs with their sisters, cousins, and sometimes, their mother.
Case closed. Hope this helps.