How is this different from the current ignore feature?
It's more robust. Blocked users won't see ANY content from the blocker and vice versa. Right now, I can ignore a user but they can still see and comment on anything I post, reply to my posts, etc. and I can still see when they post even though the content is hidden. With this, the blocker and blockee would literally be invisible to each other. Even putting their name in a search would return nothing.
Use case: if I want to share links with everyone but a few annoying asshats, I block them and they won't even see that I posted anything.