Depends. For most women its just a part of fashion and I won't lie, its not a bad thing when they know how to pull it off right. What's sad though is some girls are gorgeous without it and when they put it on, yeah they still look good, but not as good as they did without.
Good example is there's a girl names Danielle I used to work with who had a thing for dying her hair brown, wearing mascera in a way that made her look kind of pale, liked to put the eye shadow out off to the sides - it did look kinda cute in an alternative way. However she took a break, had a kid, and we didn't see her for about 4 or 5 months while she was dealing with her last trimester and taking the first few months with her son (I think). When she came back it looked like she'd been on the stairs or something because she looked about as thin as before her pregnancy but the first few days back she was working in the back of the house expediting food. Her hair at this point was that light brown to blonde, she wasn't wearing any makeup, had a bit of a tan, and looked IMO looked a lot better than she did with the make-up on (looked even more attractive and even a little bit like a hardass - 2 pluses). There was another girl named Patty who also did herself up with the eye-liner, a little too much, and when I saw her plain I really wondered why she wore it at all.
Then again that's not every girl and I can understand it from the standpoint of style - it's like I don't feel comfortable leaving the house unless I'm wearing some cloathing that I feel pimp in; I guarantee for them it works just like that; its just an extention of their own persona their trying to capture, project, and if it gives em a bit more confidence in their step or makes em feel better about how they're looking to other people - so much the better.
(edit: just to add my opinion, I think girls achieve their most with make-up when they have it applied where it looks either minimal or where it just takes their attractive features and gives them a little bit of boost. When they've got the Dead Presidents war-paint on though it hardly ever looks as good as nothing and never as good as just well-applied basics).
_________________
The loneliest part of life: it's not just that no one is on your cloud, few can even see your cloud.