Retention on your wrist. The last thing you want for a knife to do when chopping is to bounce out of your hand and cut something.
Some folders have them to help remove it from your pocket quicker. A fob on the end of a small knife can also serve as an extension on the knifes handle.
Plus 550 chord is handy stuff, so some guys try to stash it on as much stuff as possible, for emergency use.