Mold loves moisture. And mold loves to eat dead organic materials, like wood, paper, and cardboard. The bad news is most crawl spaces have lots of moisture, as well as wood and other organic materials for mold to grow on.
Mold also likes to eat dead insects and there are usually lots of those in the crawl space too. Vents in the crawl space provide an open invitation for insects and pests. Vents also let in moisture, creating the ideal home for mold.
Mold releases airborne spores that eventually make their way upstairs. Mold in your crawl space isn't good for your health or your property value. Nobody wants to buy a house with mold.
Part of keeping your crawl space dry involves a sump pump.