Molecular Sieves
Basically, these little packets are desiccants, although in the preservation world, they are known as molecular sieves. Placed in cans and boxes that store film and tape, sieves adsorb (yes, adsorb) gases, moisture, and other contaminants. Acetate tape in particular benefits from the adsorbent property of molecular sieves. Moisture provokes a chemical reaction (called hydrolysis) with acetate tape’s cellulose acetate backing. It’s damage that's easy to spot because the process gives off a strong acetic acid odor, like the smell of vinegar. This type of deterioration is in fact called vinegar syndrome, and it is "communicable" to neighboring tape. Sieves help to suck up the acidic fumes before they can spread.