why is a rating system even necessary?
On this board, in this context, a rating system as currently employed is not necessary. I don't even bother with it, personally, however, Percy is using this as a test bed for his forum software. As such, he needs to make sure it has features people want, and needs to test those features.
There are many forums out there that have a rating system, and for some, it is even helpful. For example, a forum on gardening might have a rating system so that if you ask a question about fertilizer and you get three different suggestions, the person who's been more highly rated has probably been more helpful and useful as a resource.
Remember, we are but rats in Percy's giant lab, if you're nice and make it to the end of the maze, you may even get a piece of 10-day old cheese, so buck up.