Basically, it's a throttle on your trust level.
Major moderation functions are tied directly to reputation (things like editing, voting to close and deleting). The reputation cap serves to prevent you from gaining privileges that are beneficial to the site, but can be used for nefarious purposes too quickly.
Even if you somehow managed to hit the rep-cap every single day (no easy thing to do here), it would still take you somewhere around 10 days to get free editing privileges (2k), 15 to get close votes (3k) and 100 to get unfettered delete votes (20k).
This serves to force you to learn what a community is about before you get the tools to moderate it. This is a good and important thing.