Hoppy, reading it the way you describe, all the fluid would still go out the hole in the bottom of the tank if that's where the hole was!. I am sure what you meant to say is that if there was a leak somewhere in a line (or ram or wherever else in the system being served), only half the tank contents could be lost to that system. The remainder would be preserved for an alternate system.
Another place you find stand pipes (but of lesser length) is in some fuel tanks so that any water can collect below the stand-pipe, allowing clean fuel to be drawn off through the standpipe.
And the standpipe is the basis of a fuel reserve system on many motorbikes. When the fuel gets down to the standpipe level, the engine stops unless you open the bypass valve in order to access the remainder in the tank.