Reasoning behind this?
 

Is there a reasoning behind putting an on and off switch for the fuel pump?

yes to prevent flooding. so you can kill the fuel when you suspect it might be flooded and turn it over.

Edit: i have had one installed on all of my rx's and i use it to cut out excess fuel both when im done driving and when im cold starting, ive NEVER had a rotary flood since doing this. Before i put a switch in on my 85SE it would flood every day of winter, then i had to take the spark plugs out, crank it over, hope it works, repeat. then after i put in a kill switch it was, turn it on, turn it over ,if it didnt start right away, turn off the fuel, crank again till it did start then turn on the fuel. NO PROBLEMS SINCE.

Ive owned an rx for the last ten years straight and since i started using a kill switch ive been flood free for 9 years!

I dunno about "prevent", but it does help to clear the flood...
It can also be used as a security device if you hide the switch itself.