There are some fleeting ideas which are truly great and the entire purpose of writing them down is to maintain their value. ("I was in the bath one day...") Others ideas aren't particularly genius, but may need to be kept for later use or actionability. Still other ideas are just useless and these get flushed out in the wash.

BD doesn't do an exceptional job of looking at the entirety of the spectrum of ideas here, which could be useful and illustrative, but instead focuses on moving things toward what he's calling "main notes", and even these can have different levels of value to a particular person.