I suppose there are writers who finish a first draft and move on to the next book. Louis L’amour was like that; you can tell from his goofs. There will be a statement in chapter three that is flatly contradicted by another statement a few chapters later. It’s the kind of thing that even a cursory second reading would have caught.
For the rest of us, there are always bits left on the cutting room floor. In my case, whole novels worth, but I’m probably extreme.
One thing I like about computers is that when inspiration strikes, you can write down an unrelated paragraph or two right in the middle of the chapter you are working on, and then go back to what you were doing before. Maybe put it in bold to catch the eye. Later, at leisure, you can retrieve it. I do that all the time, and with every rereading the bit catches my eye and reminds me to make room for it.
Sometimes, no matter how good a bit is, it never gets used. That offends the little voice in my head that says waste not, but there is no help for it.
I was revising Like Clockwork today when one of those bits shamed me that I couldn’t find a place to use it. For context, one of my characters, called Balfour, is a kind of ghost of Robert Louis Stevenson. In the bit that will never be used, someone says to Balfour . . .
“You wrote a boy’s book about pirates. Who knew that there was anything more than that in you?”
. . . and Balfour replies . . .
Anyone who writes genre fiction will understand Balfour’s pique at the assumption that he was only a writer of books for children.