[t0]

What was the since in ever turning it off?

[jgeorge]

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.

[DanBC]

Some software used the machine clock directly for timings. Thus, leaving the machine in turbo mode mode the games impossibly fast.

You'd turn it off to make games playable.