They want to allow forwarders (such as mailing lists) to modify signed messages all while keeping the original signed author email address. It is meant to replace ARC which is now deprecated.

You can now verify who changed what and when but it is still based on will and trust to accept what has been altered and therefore a security theatre.

It ‘solved’ a problem for a mailing list that insist on altering signed messages, even though they do not have to modify forwarded messages in my opinion and many lists do not.