POSTs are how federation works (ActivityPub is a Push-based protocol). When you “subscribe” to a community on say lemmy.ml, you are telling it to periodically send you updates about that community. This comes in the form of POSTS.
As to the frequency of the POSTs, I can imagine something like lemmy.ml having a lot of activity that it needs to inform your instance of (new votes, new comments, new posts, etc)… but I’m not sure if one request per second is reasonable or not.