I wear it because it's something easy and inexpensive to do to make myself look a bit more put together and professional. My work doesn't require me to dress up (and I take advantage of this, mostly wearing jeans and nice shirts), so I feel that grooming, including makeup, becomes a bit more important. All I need for work is to fill out my brows, put on a bit of mascara, and cover up any spots - it makes a huge difference.
If I'm going out, I like to get dolled up a bit because it makes me feel purty, especially when so many other women up here are very fashionable and made-up at all times. So I suppose there's a bit of a competitive thing too. :P