I don't understand the benefits either but this link is where I read about it. This link is a.pdf document. They state that the oxalic vapor condenses to crystals and would adhere to the upper portions of the hive. Unlike smoke the vapor does have mass so gravity would help distribute the crystals downward. I'll look some more.
They also state that the upper delivery seems more effective. I was wondering how they determine the amount of extra acid that has to be used when the application is performed with the propane powered units.
The vaporizer I built is made fron 3/8in stainless pipe and 1/4in sst tubing.