1 cubic foot = 1 gallon, huh?

why is this in the references exactly? we are talking about water here and not kool aid or milk i guess? these are both measurements of LIQUID VOLUME? can someone offer up a little bread crumb trail to a problem set it chapter in MEEB or AGS or BALLAST or CHING2X or OLINS or whatever? i’ve never done anything with water except get some idea of rainfall intensity and i am pretty sure they don’t supply this value in gallons in the US at least. any ideas? my google fu on this one is all tapped out. THANKS