One interesting use-case is append discs, or at least that’s how Konami called them. These are commercial “data discs” released after a game’s original release, that required booting the original disc first in order to then access the additional contents on the new discs. DLCs before the DL was possible, in other words.
I would hazard to guess the best known case in the West is the original Persona 3 Fes, which had both a standalone and an Append Disc version in Japan.
The cheaper Append Disc version (with the original sticker telling you to be careful about what you are buying)
As I mentioned above, I am pretty sure Konami either coined the term or made it standard in the Japanese games industry, notably with the various Bemani series ported to PS1 and later platforms.
I won’t go The Complete History of Append Discs on y’all but just another fun case I want to mention is that the PSP got them as well. Here’s the Append (Universal Media) Disc version of God Eater Burst.