Well, took a while to find but this is on the Sony website. Not what I would Call detailed!
Sony Music USA
A traditional audio CD when placed in an audio CD player, CD EXTRA offers a free interactive multimedia experience when played in a computer’s CD-ROM drive *. CD EXTRA offers the music fan a closer look at their favorite artists, with many CD EXTRAs containing exclusive content.
As already posted, They forget to mention a closer look and interactive experience of seeing your system crashing or your CD drive disappearing if you try to remove their poorly designed rootkit based DRM!