Even if your manager will mostly rely on their own judgement, the write-ups will be the only thing that future managers (at the same org) can rely on to learn a

Even if your manager will mostly rely on their own judgement, the write-ups will be the only thing that future managers (at the same org) can rely on to learn about your work, and also for things like promotion, your manager will probably need to convince a committee, so being able to draw from your self-assessment will help them argue your case.