I bet it's used to wow clients who don't know web programming. "If you want to start over, just click this link and BOOM, everything empties out." Nevermind that, as you note, nobody ever wants to start over.
Chromium* has undo. Fill out an entire form and you can undo, redo the entire form change-by-change. Not sure about the other browsers as I don't use them with any regularity
