I desperately need this. Let’s just say I can’t ever remember whether I’ve read something or not. Many times, a book will feel very familiar, and I will think I have read it, but I actually haven’t. This would be a good way to keep track of stuff like that. The only problem would be that both of those shelves would either need to be much, much larger, or I’d need about fifty of them. Probably would go for a bunch of slightly longer shelves.
Another usefulness of having a shelf for books you’ve already read allows you to go and re-read stuff you enjoyed. You can enjoy it for what it is, not simply looking for the ending. How does it affect you emotionally the second+ time around? How long has it been since your first reading, and how have you changed since then? How do those changes affect your interpretation of the book?
Pick up something you wrote some time back and re-read it. Ask yourself those questions. Re-write to make it suit your current-day self. How is it different? Did you keep the ending the same? How have the characters changed? Put it aside and put a note on it that says to re-write it in a month from now. Try it.