To ensure that by the time they're controlling the front page, they've shown that people like what they have to say and that they've proven themselves a fit for the community.

I thought that's already the case, though. Can't you not vote stories up until you have a certain karma level?

So they have time to get a feel for what's appreciated and what's not around here. If they can upvote right on arrival they might erode the culture without even knowing. Multiply that effect by X new users each day and you have a continuous degradation.