Many PS1 games stored their soundtrack as normal CD audio tracks on the same disk. The first track would contain the game data, additional tracks were music.
This was super common in early games but was almost unheard of by approximately midway through the PS1 lifecycle.
I think the biggest reason was the desire to cram more data onto the disk. Uncompressed redbook CD audio fills up your whole disc... fast.
It's also hard (impossible? I think?) to do seamless looping playback with CD audio if you're just streaming redbook because the laser has to seek back to the beginning of the track. This is a common issue in games where the background music for a level might be e.g. 1:30 of looping music.
Lastly, storing naked redbook audio on the discs may have been been seen as affecting game CD soundtrack sales, which were often a pretty big deal in Japan.
This was a fun trick, but may $DEITY have mercy on your eardrums if your volume was up, headphones were on, and you accidentally played the data track.
I think the biggest reason was the desire to cram more data onto the disk. Uncompressed redbook CD audio fills up your whole disc... fast.
It's also hard (impossible? I think?) to do seamless looping playback with CD audio if you're just streaming redbook because the laser has to seek back to the beginning of the track. This is a common issue in games where the background music for a level might be e.g. 1:30 of looping music.
Lastly, storing naked redbook audio on the discs may have been been seen as affecting game CD soundtrack sales, which were often a pretty big deal in Japan.