The only use of the "turbo" button on some PCs in the 90s I found was to slow the machine down for the few older games that didn't have code to deal with the various speeds.
That's it's intended use. It's more of an anti-turbo, but you can run in compat or fast modes. And some of them had a sweet MHz display that was just set with jumpers so you could put BS and wow your friends.
I knew a computer reseller through my father and the turbo button was just a glowing button on the front a user could turn on or off without affected the CPU speed. In other words it became a marketing gimmick and I suspect more often than not it was not functional in any way except to give the user the idea that their 386SX was in TURBO mode.
More accurately, it was real on earlier hardware when people had things like games which were unusable on processors faster than IBM’s original 4.77MHz 8086 processor. As a kid I had a hand-me-down system where that was definitely necessary for a handful of older games.
That meant almost every case had that button and people got used to it. That didn’t mean it was always connected - we’re talking a simple cable connecting to a couple of pins on the motherboard so this was trivial - and I knew a couple of people who disconnected it to keep kids or certain hopeless adults from clicking it, forgetting, and then complaining that the computer was slow.
By the late 386/486 era that’d become pretty common and you stopped seeing it as much.
I remember using turbo switch when playing Tetris. At the startup game would try to adjust its speed to the speed of the hardware it was running on so turning of turbo (basically lowering 8086 frequency from 10 to 4.77Mhz) resulted in a game being so slow that it was possible to easily reach scores over 20k.
