As you know, moisture can build up in the tubing of the aid, affecting the frequency response of the aid. It can also corrode components and battery contact points. A little moisture may not matter very much, but prolonged exposure can damage the aid.
This product is meant to be used with the Dry & Store Global ll and Zephyr models, and will permanently remove moisture. Often sold in a 3-pack, with each Dry-Brik lasting 2 months.