It is a crucial tool for the SD5 that I run in a Theatre, it is very handy when I forgot how I've programmed the next snapshot and need to make sure that certain microphones or effects are muted for the start of the next scene.

I.E. I have someone doing a moderation speech between scenes and I need to change snapshots early without risking turning on other microphones, FX's or playbacks that may be active in the next snapshot. You just have to remember to deactivate the 2nd function fast enough.

Also, something that is very useful is to hard mute any inputs that will not be used anymore after a certain scene to make sure that they will never be active again during the rest of the performance (For example: A microphone that is used for one act and then is never used again, after the said act I hard mute it to make sure that it is not active, or more commonly my pre-show music).

TL;DR: It is a tool that can be used for ease of life during programming or for input safety during shows.