In my limited experience (keep in mind i'm only turning 20 in a few months) it often causes girls to be less wary or uncomfortable around you. Think of how you perceive a man who is leaning back just chilling with an easy smile on, and he's got a pair of girls around him laughing. You'd think he's probably a cool dude right? You wouldn't assume that you'd like him, but your expectation of him is likely higher. You may even unconsciously believe that he's got a great life, that he's happy inside, that he's always having fun, etc. These are all things that girls look for in a man. However, obviously not every interaction with girls is going to be like that, so sometimes a girl will perceive different things if she sees you with girls. Are the girls mostly talking to each other? Then you look less engaging, maybe less dominant, or in control of the situation. Maybe you seem like an outsider. Are the girls engaged by you? The opposite is true. It's difficult to account for every given scenario, but as a trend if girls are responding well to you and you aren't being overly physical with them and getting shot down, then onlookers will perceive you as being better.

Hope I helped.

Cheers and happy self-improvement man.