SHATFIELD Because iTunes sometimes doesn't get it right.
I had a CD that was imported using a Linux box some time ago, which I then imported into iTunes when I switched to a Mac. Well, the MP3 files had v1 tags, but the v2 tags were empty. iTunes displayed the tracks correctly in the song browser, but when I played the song, it would add an "_" to the name and strip the tags!
ID3 Tag Editor fixed the problem by allowing me to update the v2 tags from the v1 tags.
(Version 1.7.7)