The best reason to use Phosphoric is the iron phosphate it leaves behind. Iron bonds with phosphate much more readily than oxygen, so the left over acid salts on the surface retard the formation of more rust from moisture in the air. Other acids strip the iron oxide (rust) from the surface of the metal but don't leave behind a nice coating, so any moisture causes even worse rust because the surface has been cleaned.
It is best to use the iron phosphate out in the open air because you don't want to be breathing the fumes of any acid. Phosphoric acid doesn't have much of a smell and isn't terribly toxic to humans other than the pH, but when combined with the contaminates on the metal or inhaled its bad for your lungs.
Coca Cola contains phosphoric acid as a buffer for the fizz.
