Why is water activity important in baked goods that will be consumed in probably 24-48 hours after purchase?

Most foodborne bacteria do not grow at water activity levels below 0.85, and most molds cease to

grow at water activities below 0.70. Water activity is not a kill step but a control step. Microorganisms aren't destroyed, they just can't grow enough to cause infection below 0.85. Above, 0.85, another hurdle has to be used to control bacterial growth like  temperature. For example, if you have a buttercream frosting with a water activity of .90 and pH of 7.43, it has to be refrigerated to control bacterial growth. If you leave it out at room temperature pathogens can grow to levels to make someone. sick.