Same reasons as to following game devs and artists on twitter: updates, work in progress posts, and teaser trailers - first hand news without relying on third party sites
I personally left Twitter, since it not only diminished my reach thanks to the implementation of Twitter Blue, but also was an increasingly toxic place (and it begin with being a toxic place in the first place, thanks to algorithms), which didn't bring the best out of me.
I think there might be a connection between poast-dot-org and poa-dot-st. The latter is a Neo-Nazi Pleroma instance. I haven't seen any proof that the domains are connected. Might not be.