XP is basicly an indicator of participation on the site ("monk community experience points" if you want to think of it that way)

Even if you never post, and never use the chattbox, and all you ever do is read and vote on posts you read, you are still participating in the community, because your votes contribute to the collective "community opinion" of the nodes you vote on.

So for your participation in the community (however small it may seem) you have a chance at earning "community experience points"