The pharmaceutical industry has been measuring moisture for decades. Why? Because most people think water is the enemy of API stability. Turns out they're only partly right.
Moisture is how much water is in a product. Water activity (aw) measures the water's energy level. The energy level is much more important to reactions that hurt pharma products. Let's take three famous ones: API hydrolysis, crystallization that affects dissolution rates, and caking/clumping of powders. All of these are time-dependent reactions with rates influenced by water activity. For example, USP 1112 specifically states that water activity can be used to reduce degradation of API formulations susceptible to hydrolysis.
Moisture migration is another quality problem that crops up in the pharmaceutical industry. This typically happens when gelcaps are filled. Sometimes the capsules crack while sitting in packaging, rendering the product unsuitable. This happens because the water activity of the powder and capsule are different, so moisture leaves the gelcap and enters the powder. Note that water activity is the driving force for migration - moisture content is not.
For a particular product, water activity and moisture are linked: if one goes up, so does the other. So even if you just want a better way to measure moisture, water activity can give it to you. Why?