Total Annihiliation also did this. It wasn't that uncommon in that era since audio compression was difficult and so a common process was to install off the CD t

Total Annihiliation also did this. It wasn't that uncommon in that era since audio compression was difficult and so a common process was to install off the CD to HDD, run completely off HDD, and then play audio off a series of audio tracks on the CD if it was present in the drive (plus maybe some oldschool DRM by checking for the CD).