Because your crawl space is situated directly below the living area in your house, whatever is in the crawl space — including air, odors, moisture and pests — can find its way into the house through gaps and cracks you probably aren't even aware of.
The air in an untreated crawl space is uncontrolled, allowing it to pass into your living space. If the moisture in the crawl space has caused mold and mildew to form, these are also able to pass through to the upper floors. If the air quality in your crawl space is poor, as it is in most untreated crawl spaces, it will adversely affect your indoor air quality — the quality of the air you and your family breathe.
In addition to the poor air quality of an untreated crawl space entering your living space, your heated or cooled air can also escape through your crawl space, making your home less comfortable, wasting your energy dollars and causing your HVAC system to work harder than it should.