Shared blocklists are older than social media; in particular see USENET killfiles, which were often shared. More recently, you had user-made shared blocklists on Twitter until Musk broke the API. There’s nothing particularly new about them, though having them as a convenient first-class feature is somewhat new.

They help make Bluesky usable; for instance I subscribe to one which nukes transphobes, because, really, I do not have the patience to listen to their One Joke anymore, thanks. And another which warns on people with AI-generated profile pics (these are virtually always some form of scammer, or, worse, AI evangelists).

The blocklists are not mandatory.