I reckon it goes back to the era of 2 strokes meself, when for wot ever reason the bike suddenly accelerates & you find yourself to stunned & to brain dead to use either the clutch or brakes (or both) & your just hanging on to bars for grim death (if anyone else out there every had a go on the Yamaha RZ500 V4 2 stroke, it might not seem such a bad addition for stopping). Actually I 1st noticed these on off road bikes, they then became fashionable on a few roadies & they just caught on.
Sort of like tacho's in cars, another piece of equipment everyone wanted but never used.

 

Try this scenario... sitting at a light bike in gear. Clutch cable breaks you lurch forward and grab a handfull of throttle...Oncoming traffic. Which is gonna be easier to use? Kill switch or ignition?

 

well i have used it one time when my throttle locked up on me. i was on the freeway and there was traffic so i didn't want to turn off my lights and have some on hit me. i also use it when coming home at night as well sense my exhaust is really loud.

 

Nah! It's so you can reach over just before the light changes to green and kill your opponent's engine and then blast away!

I did see a guy at a rally in England on a brand new ZX10 drop it and then watch as it destroyed itself by doing a fully horizontal doughnut on its side. I was rolling on the ground as he chased it around and around trying to hit the kill switch!

 

Obvious answer:

It's a switch that kills the engine.

 

Its good for efi bikes so you're not constantly priming the fuel system when turning the key on, off, on, off, on, off