You basically schedule your product for launch on a specific day. The benefits it provides are drumming up support for your product so it gets more of a boost on launch day. It can help it climb the leaderboard.
The negatives are that if you have any large delays, it will be a big letdown since you can't delay your product by more than a couple days. You can make it earlier by more than a couple days either, so if you end up releasing really early, then you won't be able to change that either.