The gasoline powering the development engine. Boiled down, a set of notes is a lot like an extended version of the comments page from a coverage report. Notes describe how the screenplay must change between drafts in order to get one step closer to a greenlight. There are good notes and there are bad notes. Good notes searchingly probe into a script's plot, characters and themes, pain-stakingly helping the project to become its best and truest self.
Some writers hate notes.
Some embrace them. But notes are inevitable. To get a greenlight with no notes is like skipping three grades in high school. Alan Ball suddenly became that geeky 13 year old senior who goes on to own a multi-million dollar internet startup before he can vote. Here's the funny part.