Stormin Mormon wrote, on Fri, 27 Jun 2014 19:21:57 -0400:

CY: Pump for what? Sounds like you use hydrant pressure
to fill the tank, and gravity to drain the tank into
someone's well.

The fire hydrant would likely fill the tank on the truck,
but, the homeowner water tanks are ten to fifteen (or so)
feet up in the air, and some are on hills above the houses.

So, the pump is to get the water from the truck all the
way up (maybe fifteen or twenty feet?) to the top of the
water tank, which is the only ready-made opening.