Say your in the fun situation of managing to run bone dry on fuel, aren't quick enough to switch over from mains to reserve, or if you just rebuilt your motor.
If there wasnt the prime position, you would have to crank your motor over HEAPS to get fuel drawn to the carbies, which isnt good for the battery, engine or starter. With the prime position, just cut it over for a minute or so, and voila, fuel float chambers & hopefully away you go
