The kill switch on a motorcycle is no different to the same type of switch in other applications.
As indicated by the name, the function is to kill the engine, instantaneously.
In the old days, speedway riders used an old set of contact points mounted on the handlebar near the throttle. It was connected to the ignition coil and held open by a small tag of leather which was connected to the riders wrist by a cord.
In the event that a rider came off at full speed, the tag would pull from between the contacts which would short the ignition and kill the engine, preventing an uncontrolled bike from driving into the crowd.
Many machinery systems have kill switches that shut down the machine in an instant, sometimes even damaging the machine.
They are highly visible and easily accessed so that anybody can kill a machine in an emergency.
The primary intent is to stop the machinery in the event of human danger.
The US DOT required all motorcycles sold in the US to be fitted with a readily accessible kill switch, I think this came into being in the early 1970s.
The primary reason for the kill switch at that time was no different to the speedway riders or the machinery systems.
It was to kill the engine in the event of an emergency such as a stuck throttle cable, snapped clutch cable or accident.
Our Rebel manual instruction states:
ENGINE STOP SWITCH
Should normally remain in the (Run) position.
In an emergency, switch to the X (Off) position (the starter motor will not operate) to stop the engine.
So according to the manual, if it's not an emergency, don't use it.
Whether you use the kill switch to shut your bike off, or the ignition key, or the side stand, it's a matter of choice (IMO). Just have a "shut down routine" whichever way you use.
Of course if you were a real showoff, you could just pull up outside the bar and simply drop your bike on the ground and let the lean angle sensor do all the work for you.
<joke of course>