I used to drive a service truck that had a small brass valves installed in the two gas tanks. Handy. We used to drain off a bit to kill wasp nests (A couple of ounces of gasoline tossed at the nest drops wasps on contact), for running small equipment like chainsaws or trenchers, and for filling up generators on a work site.

It was also handy for draining water that condenses in the tanks as temperatures swing. Park the truck to the taps are on the low end of the tanks in the evening and when you get there the next morning you drain off the water and sludge that fell out overnight.

If you have frequent call to siphon gas out of a vehicles tank and you happen to drop the tank for service it is easy to drill a hole and install a tube from the top. This can be stuffed down into the tank leaving the plugged end where you can grab it with your hand. Gas on tap. Probably wildly unsafe and certainly a violation of DOT standards but it is a country-boy solution.