On AT computers you had to go to low frequency to play games built for XT's. For example M1 Abrams tank simulator was a slideshow on a XT at 4.77, slow but almost playable on 9.54, fine on a AT at 12 MHz and unplayable on a 386 at 40 MHz (it was like a movie played on 10x speed). On the 386 with turbo off (16 MHz) was playable.

It's actually the other way around. You'd have to disable the Turbo to play the games. Because they where synchronized to the slower cpu clock frequency. For your spreadsheets you would use Turbo. Marketingwise it mirght make more sense to have a button to make your computer faster instead of one that makes it slower.

The games were unplayable at the higher frequency, presumably because they made some sort of assumption about the relation between clock speed and play speed. Typically the computer was on "turbo" all the time.

The turbo button made the machine slower for compatibility with older programs, as described in the parent post.

Energy efficiency/consumption would be my first thought.