The one place I find this very useful is when creating lossy versions of songs. The Sonnox Pro Codec can isolate what's removed when converting from a non-data-compressed file to MP3/AAC/etc. Sometimes it's instructive to hear how little, or how much, is actually lost during the conversion process.
 
As far as mastering is concerned...I once worked with Abner Spector, who produced "Sally Go Round the Roses." He told me about one time when he did 75 different masters of a song. He then had to get away from the song for a week. When he returned, he couldn't tell any difference among all the different versions, they all sounded the same to him. I think there's a lesson in there somewhere