Purpose of the overflow tube below fresh water tank?

IF that overflow tube actually goes into tank at bottom then it is a 'stand pipe' design overflow IE: pipe stands up in tank and level of tank must rise top of stand pipe before flowing out.

Similar to ones in residential toilet tanks

Overflows are used to prevent pressurizing tank should vent plug up, stops water from running out filler tube and lets you know "HEY tank is full" :B