That switch is like cruise control, but only for engine rpm. Its mostly useful for using the pto. Set your rpm, idle down, then simply hit the switch NEXT to the far right one to rev up the engine again. Kind of redundant when using the hand throttle. But could help.
Think about it this way: You need to start your pto at idle, always. You come into a field or plot to mow, till, whatever. You have your 'cruise control' set to whatever rpm you need (typically the pto speed), turn on the pto at idle, hit the switch (no guessing or adjustments needed), the tractor revs up, drop your implement (or whatever) and away you go. PTO sprayers, at the end of a turn, etc.
I rarely use mine, but its a nice feature that could be beneficial.
