The reasons I could give, just to reiterate:
* central archive of the best possible fonts for any SS2 translation
* opportunity to fix non-SHTUP fonts for good like metafont.fon (lacks the "ß" necessary for the German translation, lacks almost all accented characters, does not even do US-ASCII properly)
* improved maintanability/ease-of-use for developers when converting between UTF-8 and the necessary SS2 encoding (e.g. ISO-8859-1 needing no conversion at all)