One function of these canisters is that they prevent moisture condensation on films removed from a fridge or cold outdoor environment. Water condensation is a big problem, because wet emulsion swells and becomes sticky. The film can easily become unusable. Wet film, especially in the past was an excellent substrate for fungus. Swelling, sticking and fungus easily damage the film...
Films are also sensitive to various gasses, so this is protection against that as well.