Now, I understand the effects of eye-liner and I really like it when a girl can wear it well.
I imagine that make-up is used to create the illusion that one naturally looks like that all the time. I can understand this with lipstick and rouge, some people do actually have red lips and rosey cheeks.
But I can’t think of how eye-liner is ever natural looking, so where does using it come from?
Makeup has been used for a looooooooong time. The recent fashion for using naturalistic makeup is just that: recent. In other times and places makeup was used as a fashion statement, social marking, showing marriage status, lots of things that have nothing to do with emulating rosy cheeks. Here’s a page that has some information on the history of cosmetics. Sources too, if you’re curious enough to follow up on them.
It increases the contrast around the eye, which draws attention. I think raising the contrast around the eyes adds a little extra kick to our neurons that identify the eye in our facial recognition wetware.
It’s also commonly used to make the eyelashes appear fuller and thicker.
Granted, many women don’t totally get that look since they leave a gap of skin coloring between the liner and lashes (or use bright colors, etc), but it’s one of the purposes.