Ever used a PC with a Turbo Button?

Yes.

Elite was very hard to play on some machines if they didn't have a turbo button. (To turn off turbo, and put the machine in a nice 12 MHz.)

Later machines kept the button, but either just hooked it up to a display or even left it disconnected. Cases could fit different motherboards so it's handy to have the button for the older motherboards.

Yeah, and thanks for reminding me that I'm old.

Some games didn't use a timing loop, they were written with timing that corresponded directly to CPU speed. Thus a turbo mode would make the games impossibly fast to play.

Sopwith and older versions of Scorched Earth come to mind as examples of this phenomenon.