A lot of people think that air sealing the crawl space is specifically for the crawl space itself. Due to a process called the Stack Effect, the air from the crawl space actually gets pulled up through the home through the lower and upper floors until it is finally exhaled out the top. This means that the air in the crawl space becomes the air in the kitchen, which in turn enters the bedrooms up above.
Mike Rusk, owner of Ohio Basement Systems, explains the stack effect in greater detail. When warmer air rises up, it pulls other air into the space behind it, which reinforces the reason to have a sealed crawl space. If there are open vents into the crawl space, as the warm air tries to escape upwards it will pull the outside air into the crawl space. When you're experiencing 80-90 degree weather and 90% humidity, this damp and humid air will flow right into the crawl space and up into the home.
This humid air flow, when coupled with the organic materials in the crawl space, promotes the growth of molds and fungi which now have the perfect environment to feast and thrive. This is what typically causes musty smells coming from the crawl space. Seal your crawl space and protect against the health risk posed by mold growth.