For me, it's a confidence and self-pride sort of thing. I feel a lot better when I have the ability to look nice. That, and... You know prettier girls have more friends, get more attention, etc, etc, etc. -_-
Girls are bombarded with images of what it takes to have "value" - even if it is superficial - clothes designers, the cosmetics industry, cosmetic surgeons / dentists, etc. all make money off of making girls/women insecure abut their attractiveness - which is now related to social status. Pretty girls have more doors opened for them - that's the message they get constantly - and what drives them to beauty salons and to buy cosmetics. These businesses dehumanize females to become objects - to compete with other girls and to attract more males.
On an instinctive level, probably without realizing it, they are trying to look healthy. This is because, on that same instinctive level, what we look for in a potential mate (even if we consciously are looking for a casual date) is good health, to produce healthy offspring (even if consciously we don't ever want children). So -- clean, shiny hair = health. Bright, clear eyes = health. Good weight = health. Good posture = health. Clean, clear skin = health. White, solid teeth = health. Good coloring = health. Girls/women are more concerned about their appearance than boys/men because: (1) Boys/men are visually oriented; what they see is important to them. (Not saying your shallow, guys.) (2) Magazines, movies, and advertisements present girls/women who seem to be perfect (mostly through makeup and photographic enhancement) and the girl/woman in real life feels she must compete. I know, long answer.