Because lots of people use Grouping for other things than just a classical Work, like grouping music by Mood, Country, Label or Subgenre, and Apple seemed to want to retain that functionality.
Apple have now (correctly) reduced TIT1 to the more strict ID3v2.4 definition of Work (which allows them to tag classical music better), while their wish to maintain the original functionality of Grouping has resulted in a new field GRP1. I personally think that’s a well thought-out way to deal with it.
The ID3 specification authors should have also made a field for Lyricist and Producer–two important people who often determine whether or not you will like a song.
They did, although few music players support them:
TEXT (“Lyricist/Text writer”)
TPE4 (“Interpreted, remixed, or otherwise modified by”, effectively you could call this “Remix Artist”)
TIPL (“Involved People List”, a list of roles/names, which could Producer, Engineer, Coffee Boy, etc)
