How to slow down a program and why it can be useful

For the Commodore 64 there was a product called the C64 Snail which could slow it down.

Later on the early PCs we had a Turbo Button, but since everyone had it in Turbo mode all the time it essentially was a way to slow down the machine.

EDIT: Found an image of what I remember as the "C64 Snail". It is called "BREMSE 64" (which is German for brake) in the image.

https://retroport.de/wp-content/uploads/2018/10/bremse64_rex...

The purpose of the Turbo button on PCs was indeed to slow it down, originally to the 4.77 MHz of the original 8088, for compatibility. For marketing reasons it was designated “turbo” instead of something indicating a slow-down. Turbo mode was always the default, since that was the standard speed of the respective CPU.