Dave,

The pump provides reliable fuel delivery to the carburetor, and also allows a larger venturi than straight suction will allow.

When the engine was developed, a .61 was the larges 2-stroke engine allowed in FAI pattern, which is what the "Hanno" engine was designed for. You wanted every scrap of power you could get out of the egnine/pipe combination.

With the O.S. Hanno pipe, and an APC 12x12 prop, we'd usually see static RPM around 9,600 or so. This is where the engine was designed to run.