I think there are as many different reasons for having the reputation thingie as there are people participating in the discussion.
There is one problem on this site which is fairly obvious, though, and that is that the knowledge of the posters varies widely from topic to topic. Now, this would be perfectly fine if those who know little would keep their mouths shut and ears open when the discussion is on a topic where they know little. However, this often is not the case, and if a newbie to the site chooses to follow the wrong advice, it can cause problems. Among other things, Jeremy is interested in developing a mechanism that will permit some measure of a person's knowledge in a specific area, and this reputation system is a first cut at that.
Personally, I'm ambivalent about it. I think that it is a good idea to have people who give advice rated according to some set of criteria so that people asking questions will have some idea who to listen to, but I can't come up with a good mechanism to do that. This reputation system is Jeremy's attempt to come up with a good way.
More power to him. Personally, I will neither try to game it nor try to advance in it. I will just wait and see if it works in the desired fashion.