Your Mastodon instance is caching stuff locally. If you post something to your instance it lives there until another instance requests it. Then it lives on both instances. If your instance deleted the toot/content and I try to access it after the deletion I won't be able to get ahold of it. Same concept as IM2000.

Note I'm not sour on the idea of IM2000. I genuinely think its ideas were used by ActivityPub.