While you write, you accumulate many many pages of writings. Some of them you use and some of them you don’t. You may even keep a journal with your ideas in it. Or, you might also have a three ring binder you keep notes and writings in. After some months or years, you publish a novel or two. What do YOU do with your notes and writings after you’ve published your book(s)? Do you keep them? Do you throw them away?
If you throw them away, that’s your choice. You are well within your rights to do that. BUT, that means later you can’t go back and look over them when you need inspiration. Yes, inspiration. Let’s say you’re in the middle of writing book 3 and you are stuck. Will you have those old writings to draw from? You never know what might get your “gears” going again.
Keep those old writings. They will help you.