You would cut these if you needed to recover a man overboard quickly as you may not be able to recover them under the guard wire especially if they have a life jacket and bulky clothing
once put a foot between the deck and lower wire and ended hanging over the side upside down tethered by a wire behind the knee. i was carrying a knife and managed to reach up and cut the lashing.
Or when, if like my own, thay have been exposed to UV degradation, cut them and replace them. Most owners (self included) expect lanyards to go on and on, in reality they need to be changed every couple of years, otherwise thay could just fail when you need them most.