In the past, there was a theory that it was necessary to create airflow to keep a crawl space dry - that's why crawl space vents came into common use.
But as more and more vented crawl spaces experienced water issues and all of the accompanying problems (mold, wood rot, etc), it was understood that with air flow comes water flow, debris flow, and a steady flow of bugs and rodents. Basically, a crawl space vent is similar to an open window and leaves the crawl space - and your home - entirely unprotected.
Part of our encapsulation process involves installing airtight covers on crawl space vents to prevent them from allowing water and debris into your crawl space.