I too am a Harry Potter fan, and my belief was that the kids had to study Astronomy, to possibly get them ready for astrlogy, but also because some ingidients, and certain potions have to be picked or brewed based on the phases of the moon, and if you are learning about the moon, why not learn about the solar system.

It could of course just be that Rowling likes astonomy and thus used that as a subject. Before you argue that if she likes it she should be better, not neccessarily, I enjoy it, but the only constalation I can ever make out is Orion.

We may not have seen the importance of the class yet, it may be something we dismissed as trivial that will play an important role later on.