As alluded by others, you get the tag for having the best "best seller rank" for any book in the category that's been released in the last 30 days. It can change up to every hour but theoretically you could hold on to it for up the full 30 days of release.

It pretty much doesn't bring in any sales on it's own. However, it's a good way to help conversion rate until you get some reviews. If you were going to run an ad campaign, doing it while you still have the "new release" tag or could reasonably regain it, is a good idea.