Microbes sometimes get a bad rap. Good bacteria get lumped in with scary ones like botulism and salmonella. The unmistakable tangy flavor of a good salami is a product of fermentation bacteria. Reactions like this show that making good food is a balancing act - if your water activity is too high, your product will eventually succumb to the bad microbes, but if it's too low, the good ones will never work their magic.
Salt pork is the original water activity experiment. Hungry cave men who dreamed of traveling far and wide had a problem: even cooked meat spoiled quickly, limiting travel options and general happiness. Rubbing salt on the meat solved the problem, and soon they set out with portable protein strips in tow. Though they didn't know it at the time, the polar salt ions pulled the water activity so low that nothing could grow on the pork strips.
Today's producer of jerky or other cured meats is a little more sophisticated. Manfuacturers must show the FDA that their shelf-stable meat product has been dried enough that its water activity won't let bacteria grow. This presents another problem: overdrying meat can kill profits and product texture. For a product sold on weight, even overdrying by 0.2% can add up to tens of thousands in lost profit over time. Water activity helps you tightly control the moisture, keeping the FDA, your customers, and your pocketbook all happy.
Some shelf-stable meat producers still measure moisture because that's what they've always done. This practice is becoming less common as the understanding spreads that water activity is the only moisture-related measurement that can be used as a Critical Control Point in a HACCP plan. With a water activity meter, there's no sample prep, drying programs, weighing pans, balance calibrations, or training - just slice a piece of your product, put it in a sample cup, and get your reading.