For those damned yoof amongst us:
The 'turbo' button on PCs of old was a button that, when activated, had the PC run at its full rated speed (generally 12 or 16 - or 20 if you were posh - MHz). When de-activated it would slow the PC down, preferably to 8MHz (though it often just halved the clock frequency).
Why would anyone want to slow down their PC? Well, some games back then, instead of using the PCs clock to pace themselves, relied on the processor speed for timing, so if you tried to play your game with the Turbo button on, it would run rather faster than you might find playable!
Why 'Turbo' on by default and not 'Slow' off by default? Marketing, of course.