I encountered many times now on wikis (usually the small wikis) that there are 100s of local MediaWiki pages with translations from before translatewiki.net covered that language.
And even after twn covered it, there may have been local improvements that over time make it into twn, but then the local page stales.
To avoid translations from going out of sync (the longer we wait with this, the harder it is to clean them up as the difference between an override and an outdated translation becomes harder to tell), and also to avoid migration issues when messages change.
Lets run this asap.
EDIT: deleteEqualMessages.php has been completed on the following wikis: