There are many reasons why women wear makeup, here are some:
(warning: rather unfunny but highly informative tl;dr ahead)
1. Confidence issues. Women are far more scrutinised for their appearance than men are, both by potential partners and also just culturally, in general it's traditionally been the woman's role to "be pretty". Of course, not all women think they are that pretty... hence, makeup. How attractive they actually are is besides the point (and highly subjective, anyway). Some women are better at resisting the cultural conditioning than others, just like some men are better at resisting peer pressure to be or do certain things than others. Women who refuse to wear make-up are thus often more inclined to be confident and individualistic. Once again, this has nothing to do with "attractiveness" or lack thereof.
2. A rite of passage. When growing up, wearing makeup is seen as something that women do as they approach adulthood, like wearing a bra, or like men's voices breaking. It's a defining marker of adulthood. Women who wish to "grow up faster" may wear makeup at an earlier age as an aesthetic signifier - in other words they're saying "I'm not a little girl anymore, I am a woman, I have a mature outlook". (Whether they actually do or not is besides the point.) It can be a way to fit into the "grown up world", but if no-one else is wearing any in the immediate social scene it can also be a way to assert individuality. Which leads nicely to...
3. Flirting/attention. Whether men actually find it attractive or not doesn't matter, heavy lipstick, enhanced eyelashes etc still sends out a message that someone is "out to impress" and may be open to (the right kind of) offers. Heavy makeup can be a way to stand out from the competition and get noticed in dating but also elsewhere. That's why musicians of both sexes tend to use it very heavily. Do they look great? Sometimes they do, sometimes they look downright ugly. But you notice them, therefore mission accomplished.
4. Enjoyment. Some women who don't buy into the whole "all women must look a certain way to be beautiful" bullshit that our society revolves around still apply makeup simply because they enjoy it. They enjoy experimenting with different looks, seeing what works and what doesn't, etc. They treat their face like an artist treats a canvas - a space in which to be creative and have fun.
So it's a mistake to assume that a girl with lots of makeup just wants to pick up guys or thinks she's ugly and is hiding behind it or whatever. Sure, that MAY be the reason for it, but there are also other reasons why people do it.
Having said all that, I'm a guy and I don't really like the way women look with makeup on. I prefer none. My current girlfriend tends to not wear any makeup at all and I'm pretty damn happy with that.
