To steer the release, you need to track what's done, how fast you are going, and how well the system works.
At the release level, there are only a couple of interesting things to know: when are you going to release, and what will you have when you do so.
Often you have a specified date by which you are supposed to release your product. By controlling scope, you can steer your project to release the best possible product by that day.
Sometimes, you have a minimal level of capability that really must be there before you release. Even in these cases, you can usually release earlier by judiciously controlling the ...