Your crawlspace naturally communicates with your home through your floors! Although there may be insulation, flooring, carpeting etc., this does not create an air/moisture tight seal. Most homes are under constant negative pressure due to leaky ducts, bathroom vents, kitchen vents and more. This means that your home is consistently pulling make up air from any area it can. And what's the easiest place to steal air from, you guessed it, your crawlspace! Soil gasses, radon, moisture, mold, mildew and much more are found in your crawl. You don't want this getting into your home.