Lets say I have an account on lemmy server A, and with my account I make a lemmy community. Some people post in it. Everything is cool. But for whatever reason the admin/owner of the lemmy server your lemmy community is on decides to ban you. What happens to the community you made? How does the lemmy software respond to it? Does your community get banned with you? Or does the community just get stuck without any admin and people can still post in it?
It completely removes them from the database instead of leaving a tombstone in place which is usually preferable in a federated network. The purge option was mostly added for legal compliance reasons.