Interestingly that's the exact PC my dad bought us when we were kids. I loved how computers back then had a turbo button, but I could never figure out why anyone would turn off turbo! :P
I believe it was due to programs (mainly games) that relied on clock speed to work. If you ran some of them with turbo, they were so fast they were unplayable.
An example from Fabian Sanglard's game engine black book, wolf3d, p222
When the AdLib was released in 1986, developers were instructed to send data "as fast as
possible". At 4.77MHz, a PC was unable to out-pace the AdLib. Yet as CPUs got faster,
issues started to arise and the card was unable to keep up.
So if an old program ran on a new machine, it might mangle the sound or even cause a hardware crash. So the only way to play it was to slow down the new computer to the speed of the old one.
