I don't think of makeup as fundamentally different from any other activity meant to enhance one's "personal style," an outward expression of taste and aesthetics. That is to say, I think makeup falls in the same general category as fashion/getting dressed, home decor, etc. On some level these things can all be "artistic" – they are skills, one makes choices. On the other, there's signaling involved – it's directed externally (most people wear different clothes around the house than when they go out into the world), and it's a way of manipulating how you're perceived.
So the short answer is, I wear makeup because I like the way it changes how I'm perceived. But I also think it's fun in the same little-girl way that coloring is fun, that crayons *in themselves* are fun – it's like coloring but on your face. A subtle form of costuming that, as women, we're allowed to do every day. I think it's too bad that it's not culturally acceptable for most men to wear makeup in public; if it were, at least some of them surely would.
[To expand on this and address the subtext more directly:] I don't believe wearing makeup is inherently anti-feminist or the result of insecurity any more than these other practices that rarely get labeled or questioned as such, like putting effort into how your apartment looks. It's all "style" which is (almost) all signaling. My own approach to "style" is that I like indulging in it up until the point that it stops feeling fun or starts feeling prohibitively expensive. Everyone has their own set point here so I'm sure there are people who would perceive the amount of time and money I spend on "style" to be excessive, but it doesn't feel excessive to me.