To attack the physical or scientific underpinnings of astrology may well be warranted but it misses the point. Even those who believe in astrology, in a metaphysical, religious, or spiritual sense, do not engage in it as a result of metaphysical thinking. It is an identity play. It is a way for people to explore themselves, to ask questions about themselves, and to grow and change and think about the kind of people they both are and want to be.
This is why, as Manu says, it is so appealing to the queer community. People of non-normative identities are not necessarily forced, but at least encouraged to explore themselves in ways people of normative identities are not. My identity is normative and stable, and so astrology does not appeal to me. Whether I believe there is any relationship between the accident of my birth date and my personality is immaterial: I have no need of this source of identity exploration. For many (of course not all) queer people, astrology offers one among many methods of trying to explore oneself.
Neither does this need to be selfish. Self exploration does not inevitably lead to self absorption. One need note only the frequency of “compatibilities” in astrological thinking to recognize this. Astrology is as much about how one relates to others and society as it is about who one is. (This is inevitable: we are only “identities” by being distinct from others. A person who spends a life in complete isolation does not have an identity, at least not in this sense of the term.)