I've been a writer for a very long time, and I've known many others. So I've picked up a lot. Sometimes you can make a new thing from old scraps. Other times, you want to see how far you've come and it's useful to have things to look back on. Or you rewrite one sections for a third or fourth time, and realize that version 2 of a different section goes with it better, so if you haven't kept the earlier ones then you have to try reconstructing it.