I think the main appeal is that it keeps property values up and the neighborhood looking clean. Non-HOA neighborhoods run the risk of people doing whatever they want which drives people away from buying houses in the area.
I've never understood the popularity of HOA's in America. I can certainly understand the benefit of pooling resources to help with upkeep but all too often HOA's are abused by manipulative and egotistical people to impose overly draconian rules. Personally I think that political signs are horribly distasteful and wish that people didn't have to be subjected to them but what's distasteful to me could be very important to someone else; some people don't like Christmas lights but that doesn't mean they should be banned altogether.