I believe they are saying that because when you install the printer cartridge the drum door is opened. Taking it out and inserting the cartridge into a black bag and air tight is to keep out the moisture as much as possible to remove the possibility that the toner will end up sticking to each other. Toner is made up of wax and iron particles. I surmise the iron particles may corrode with moisture and the toner will end up clumping up. This can damage the printer.
Just my thoughts.