The ; splits the artists automatically when the tag is read.
But it used to be that there were other things like "&" that, when you opened the splitter and manually hit the "split" button on an artist with one of these connecters in their name, would throw the artists down into the entry boxes as separate artists while maintaining the "&" (or whatever) version of the name in the display artist box. It was handy.
Right now, it looks like "&" doesn't work (I'm not sure that it ever did, but if Tybot says it did I believe it), the word "and" doesn't work, and "/" doesn't work, but "," does. Those are all I tested.
I think I remember this being a requested change that the wishlister assured us made total sense and wouldn't mess up anyone's process
I may be wrong on that, but it sounds familiar.