Why do girls wear make up?

ive been with my girlfriend for 2 years and she has never worn make up before, and she's turning 22 this year. a few days ago, i saw a couple of girly stuff in her bag. i mean its cute that she's growing up. and she made me help her use the mascara. i'm fine with the mascara and lipgloss. but im worried she might put much more than that on her face like some girls do. do you think she would? but real qn is why do u girls wear make up? is it really necessary? do all girls go thru this? tks.