Cutting off the forward growth allows you to internalize the piece (as directed towards a single performance), making it a part of you. It's like you quit trying to go higher so you can go deeper. This makes your performance masterful and creative. You want most of your performance to be automatic, natural, organic, not something you are striving for and having to remember. The fluid grace of a baseball pitch; this is why you practice, this is how you should think of your practice - as taking as much of your music as possible and making it a natural, flowing part of you. This is what impresses an audience, the perception that what you're doing is real.
Creativity and boredom are brothers. You start thinking of new things to do when you've worked something enough to start being bored.