lmao and Andy's response tells you all you need to know about why.

It is an abused function used by many violent extremist accounts to prevent outsiders from reporting their tweets that call for violence, or more recently, to prevent people from writing a @CommunityNotes fact check. With the use of third-party blocking scripts, they can mass-block hundreds of thousands of people.

They want to be able to swarm people and get them banned from Twitter.