There used to be CD-ROMs called Red Book Audio discs (because the original specifications for CDs came in different colored books and this one was red) that had CD-ROM data as the first track and CD-Audio data for the rest of the tracks. This is what allowed DOS and early Windows games to have CD quality audio at a time when MIDI and FM synth were the primary computer audio sources (the PlayStation and Saturn also could work this way, later consoles didn’t use CDs specifically). They would just play the CD through your speakers while you played the game. For most of them this meant you could change the disc and have it play different music (but always the same track number it would have played anyway).

There was a small problem with these discs though, if you put them into a non-computer CD player to listen to the music and accidentally played the first track you may blow your speakers. This actually happened to me in my mother’s Windstar (late 90s mini-van) with the Descent disc. (Or maybe Descent 2?)

TL;DR - The ability to read an ISO track as audio or data is a quirk leftover from the original CD specification.