Any music or other content worth adding to your iPod or other mp3 player. It's not great enough that you would buy the CD and put it on your shelf, but it's decent enough to occupy some hard-drive.
I don't like the whole CD, but it's got some decent podfodder on there.
An album that isn't worth hanging onto in its bulky physical form (cd, tape, album), but it's worth loading into your iPod (in its entirety, or selected songs) and keeping electronically.
After going through my old CD collection, there were some CDs I decided to keep, but some were just pod fodder: I downloaded the stuff I wanted to keep, then sold the disks as used.