For a particular product, water activity and moisture are linked: if one goes up, so does the other. So if you want a better way to measure moisture, water activity can give it to you. Why? Water activity meters are much more sensitive to changes in moisture than most moisture content meters. This is because on dry tobacco products, a tiny change in moisture creates a large change in water activity. When compared with AOAC approved moisture methods like oven drying, water activity gives you about 10x more precision.
“Wet” agricultural products are susceptible to fungal growth until their water activities are brought into a lower range. Usually, this means getting your product under the water activity level of 0.7aw as soon as you can. Measuring water activity will always tell you if your product is at risk, regardless of which product you’re measuring.
Tobacco products are regulated by the FDA, and water activity is the only moisture-related reading that is accepted by the FDA as a critical control point.