You'd rather over-deliver than coast.
The brief said good enough. You agreed out loud, and then made it better anyway.
People assume you're angling for the promotion. Often you just can't stand handing over something half-finished.
You raise the floor for everyone around you — work goes out better than it had to.
You can't easily tell the difference between work that deserves your effort and work that merely asked for it.
… goals are visible and effort gets noticed by someone who matters.