1) It's in the standard. If you call a function iso8601 you better follow that standard. Or document where the function deviates from it.

2) Just because you didn't come in contact with ordinal dates doesn't mean others don't need it.

3) The article has a link to https://en.wikipedia.org/wiki/Ordinal_date. The wiki page doesn't have any specific use cases but just use your imagination.