You can read FEP-1b12 to understand how Lemmy federation works. To make it short, these Announces are to let community followers know about all the posts, edits, votes etc which are happening in the community. You can check the audience field to distinguish from Mastodon-style boosts.
tbf Lemmy's behavior is documented and standardized (https://codeberg.org/fediverse/fep/src/branch/main/fep/1b12/fep-1b12.md) it's just that their fallback code for instances that don't federate the Lemmy way also boosts the target posts for each update as opposed to just once on creation like you'd expect