That's precisely what it was for! In the era just before said games which would attempt to calibrate their delay loops against the realtime clock, games were only expected to run on a single speed of CPU and used hard coded delay loops to achieve their desired speeds. When someone got a new computer that ran twice as fast, the game ran twice as fast too. So as a stopgap, they gave you a button which underclocked your fancy new 386SX from 20MHz to 12MHz or whatever.

(In looking up the clock speeds since it's been that long, I also learned that some 386 chips had bodgy 32-bit multiply logic and were sold as "16 bit", and due to the weird way humans assign value, apparently these are now valuable collectibles! Source: https://en.wikipedia.org/wiki/Intel_80386#Early_problems)