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...