From spagetti to Special-K, cereal and grain-based products are a huge part of processed food. Many food companies have used moisture content as a quality spec for decades. They don't measure water activity if their products are thought "dry enough" that mold/bacteria aren't concerns for them.
So why measure water activity? Because it's more precise than measuring moisture. Having more precision may not sound important until you calculate how much you'll lose when you buy ingredients with too much water left in them. Water activity is an easy way to keep an eye on this problem, and can also give you moisture readings at the same time.
The right texture can make or break a breakfast cereal, literally. Water activity can help you know when loss of texture may become a problem for your product. This is especially true for products that have more than one component, like raisin bran. Water activity will predict which components will gain and lose water, and whether your final water activity will deliver the texture your customers want.
Cereals and grains, especially whole grain products, are often billed as "healthy". In addition to being a good source of dietary fiber, whole grains contain an array of B vitamins. Or at least they did until they sat in a warehouse for a month before getting to the store shelf. Water activity is a better predictor of vitamin degradation than moisture content. There's a simple way to link vitamin-loss to water activity: