Well, some girls a just so ugly on the inside that they can't go on living without being pretty on the outside. The feel the need to attract attention and be the most beautiful girl in the room, turning heads and not looking ugly, so they'll put massive amounts of makeup on to simply hide behind. Not all girls are like that, though.
Others just feel the need to look nice to others and it helps keep them calm - feeling ugly around a midst of attractive people is enough to lower anyones mood, so some girls like to go one step further than others by putting on ample amounts of makeup believing it will make them look prettier or simply blend in.
Others just wear it because it's nice, you know? Putting some eyeliner on or blush, maybe lipstick and going out for a night makes girls feel special and happy about themselves inside, and that's good. One night of enjoyment simply by wearing lots of makeup can potentially leave a mark inside her mind for the rest of her life.
And of course you also get those slutty girls who wear lots of makeup in the hopes that it'll make a lot of people, largely guys, stare and drool all over her, while thinking about ripping off her clothes, and so on. Attention seeking whores doesn't go amiss, here.
Others wear it to look nice for their boyfriend. They do it out of love, and choose to wear so much to look good to him, perhaps out of inferior feelings thinking he'll leave her or not love her so much if she doesn't look pretty for him...or she does this to stop him from looking at other girls, again by attempting to put flowers to shame, and such.
It can also be worn in correct doses to hide what girls think are defects - such as freckles or spots, and such. Again it ties into looking pretty and blending in, and not standing out as some ugly freak no one wants to talk to. Girls, especially the young ones, do so enjoy being paranoid about their looks.
Some girls just experiment, and chances are what you might see as a freakish amount of makeup or a hideous combination is just them trying to work out what it's like when applied to the surrounding people in her life, and stuff. They don't like societies misconceptions stop them from expressing themselves, anyway.
Despite all this, I still think the best types of girls are the types who don't need makeup to look pretty, and therfore don't choose to wearing much if at all. My girlfriend wears none, and she still manages to look like the most beautiful thing that walks this planet, to me.
It's all a matter of opinion. Those are my thoughts and opinions on the matter - I hope they make sense. :)