In a word... Confidence.
Some women wear make-up "everywhere" and all the time because they feel self-conscious and almost 'naked' without it, unless we've got a nice tan. Where we get these ideas from, who knows? We look in the mirror and say "Oh god look at the state of me, better put my face on" kind of thing, and when we are made up we feel better, like all of a sudden we can face the world.
If I'm slopping around in jeans and trainers I don't usually wear make-up but then I also feel a bit low in general at these times. When I make an effort to dress nice and wear make-up it makes me feel better about myself.
A friend of mine a long time ago wouldn't even open her front door until she had all her make-up on which I thought was ridiculous, but it was obviously like a security blanket. She didn't feel confident in her own skin so had to add a few splashes of colour to it.
Women eh?