Your scrobbler might have decided to scrobble every song hundreds of times, and you can't really remove those scrobbles efficiently. Or you might have accidentally installed multiple scrobbler extensions at the same time - wondering why multiple scrobbles appear for every song played at a time - and you want to clear them after finding the issue.
Using this script still doesn't necessarily make the process quick, since Last.fm only displays a limited number of scrobbles that can be removed on each page of your library, and also rate-limits how many pages you can load in quick succession. The difference from the implementation of @sk22 and its forks - which this UserScript is derived from - is that you don't run it manually on every page. You set your options once, click START, and the rest of the process is automated: the script works its way back through your library and stops automatically at the page you choose.