One reason to support this sort of feature is that it makes it easier to split changes into two reviewable steps: first make a minimal change that introduces some ugly thing like break-with-label, then clean it up with a no-change-of-behaviour commit.
Like with GAT, I think one of the major reasons to support this feature is because it feels consistent. "I can put generics here, but not there? I can break here but not there?". I've run into "accidentally" writing the equivalent of GAT before just because I had expected it to work.
