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Interesting.

I had the impression, the turbo button was created to slow down new PCs, so they could run old software that relied heavily on CPU speed.

Yes, originally it was added to slow down faster XTs to exactly the 4.77 MHz of the original IBM XT.

With the AT it usually slowed down to some arbitrary frequency and it was more like a gimmick.

Afaik it was to interface with bit banged hardware that couldn't communicate at the faster rates. One would turn off turbo to print.

That is why I think it made sense when it switched to exactly 4.77 MHz but the arbitrary frequencies introduced later were just a toy.