Managers, especially if they have a large span of control (but even if they don't), aren't all uniformly disciplined at recording every instance of impact. To help your manager and reduce variability (i.e. their perception of your performance should be based on data, not vibes), it's to your advantage to keep your own list.

I've had good managers that I keep in the loop with weekly 1:1's but come promo time, even they need help figuring out what I did over the span of a year. (to be honest, if I hadn't written it down, I don't even remember myself)