It is definitely a competition between girls... it's really disgusting actually! BUT it's not only that, women feel like their self worth relies on how pretty they are. Why? because everywhere we go we see the beautiful women being admired and treated well... they seem to be more respected (not real respect necessarily). Men will open doors for them, smile at them, and treat them "well". Other girls will look up to them, and wish to be like them... this is power. Women want to know that men are coming up to them or checking them out to assure themselves that they are "special" or better than the next girl because it means that they are a more worthwhile human being (or so some people think). That they are important and liked.
For men, their self worth may rely more on money and status. Now what you were saying is that all you want to do is take the guy home and f*** his brains out.. so it's likely that money and status don't matter to you... well your attractiveness probably wasn't that important to him because all he wanted was to have you for one night. But let's pretend that the both of you wanted something more than one night, you may be more interested in what he does for a living and his ability to provide. On the flip side, if he was thinking about actually dating you, he may be a bit more concerned with your appearance. A one nighter means that you can get your fill of the opposite gender without caring at all about many things you would care about if you were to take it more seriously.
But anyway, my main point here is that a woman cares about her appearance because she wants and needs to feel good about herself and that typically relies on how others are treating her. Women are social and are sensitive/sentimental people so naturally they want to be liked. Culture today is SO driven by appearance that even men are starting to worry more about how they look. And even though it's the worst now and days, we all know in the history of the world that a woman's beauty is incredibly profound!