@Snessy A lot of it was down to evolution. Amiga and C64 games often had the choice between music or sfx, it wasn't common have both in a game but does happen assuming that the music wasn't restricted to the title/loading screen. The toggle was also there for Spectrum and Amstrad games but not as common as some barely had any sfx at all (plus the Speccy only had the AY chip for 128K models). This also made it to Mega Drive games as well but music or sfx was down to the sound engines, some were more restrictive than others such as the Sound Images engine that also happens to be one of the worst on the console. Further down the line when it became less restrictive, it changed to the option of disabling one, the other or both.

Also I imagine that the programmers themselves or testers later down the line when companies started using them got fed up of hearing the sounds all the time. It is a bonus that these kept in, especially in situations where you're sneaking to play a game.

Another theory that I have as well is that sound takes up more cycles so disabling sound can speed up the game or have less slowdown in the days before redbook audio/streams. This same reason was why some games didn't have any music past the title screen, it was just too processor intensive (also why the MOD playback in Toy Story GEN was just restricted to static screens or sampled music on Atari ST). Assuming that it wasn't done by choice like a simulator.

At least it comes in somewhat handy today for streams especially when it comes to licensed music.

always wondered why a lot of old western developed games gave you the option to completely turn off music or sound effects without any volume options for them. like why'd you wanna do that lol