Why should the harvested seeds be stored in air tight containers?

I'm not sure if I understand your question correctly. I dry a LOT of fruits and vegetables, as well as can and freeze. When I dry items, I STORE them in air tight containers. This allows you to keep them longer. If they were out in the open ( in a bowl on the table . . . ? ) insects could get to them, humidity in the air could cause them to spoil, and the sunlight could cause them to fade and lose nutrients. So storing them in air tight containers allows you to keep them longer.