What you call the second purpose is often the only purpose. Or you have to talk to half a dozen other systems each of which have different constraints. You wouldn’t want to present the user half a dozen fields just so that they can choose the nicest representation of their name for each system.

That being said, in Japan it’s actually common to have two fields, one for the name in kanji (the “nice” name), and one in katakana (restricted, 8-bit phonetic alphabet, which earlier/older computer systems used and some probably still use).

You usually don't have a dozen, just two or three and if you do have a dozen, there is a certain pattern, or at least the common denominator for the half of them to be ASCII, a another half using some kind of local convention you totally know how to encode.