Water activity means energy. Energy means life. For microorganisms, water means life because if there isn’t high enough energy around them, they can’t transport water across their cell walls. If they can’t do that, they can’t grow. Water activity gives food processors an absolute measure of whether mold, bacteria, and yeast will grow in their products. Even the FDA thinks this is useful – if your product is above 0.87 water activity units, it’s a “potentially hazardous food”, meaning you have to use temperature or pH to prove your product is safe.
Note that water activity can’t tell you if your product is contaminated – only microbial testing can tell you that. Water activity testing, used with Good Manufacturing Practices, can significantly reduce food safety concerns.