Which application of hydrates best relies on the anhydrous compound's ability to both absorb water and retain water?

The hydrates are molecules that have absorbed water from the environment.

Some anhydrous compounds are hygroscopic, which means that they absorb humidity (water) from the environment.

When an anhydrous compound absorb water it transform into a hydrate compound.

Then the obvious application of an ahydrous is to control or decrease the humidity in areas where it may cause the formation of mold or other damages to the surfaces (walls), packages (cardboard), or products (wood or food) contained in the area.