Proberly to make sure the fuel which you are draining comes from the tank side and dosn't cause airlocks in the fuel side of things in the engine. H'mm but on a cessna that would work if it was turned off it would only come from the engine side.
Its just a guess though never done it myself on PA38, PA28 or Cessna types. Thats not to say i shouldn't have been doing it. Also the last time I moved a cessna fuel selector the bloody thing broke so if it is selected to both it would stay there until i noticed there is a mis match in fuel in the tanks (never happened yet) then the engineers would play with it not me. If it was a machine I wasn't flying everyday I would make sure it was free prestart then if it breaks its someone elses problem and no arguments about paying for 5 mins on the tacho.
MJ