It's used to build the diffs. There's a test in [models/edition_test.rb](https://github.com/alphagov/govuk_content_models/blob/master/test/models/edition_test.rb#L751) which asserts that each subclass of `Edition` contains a method called `whole_body`. 

Showing diffs only on `body` is a crude first-pass. We should visualise differences between simple smart answers better, but I think it's outside the scope of this story.