In my area, (with generally low humidity) foundation vents allow moisture from soil to evaporate from air and be vented to the outside. High moisture levels in the crawlspace build up near the condensation point and can affect floors and building components and promote mildew and mold growth. If you live in a very humid climate, vents can allow moisture laden warm air to enter the cooler crawlspace where it condenses and actually contributes to moisture. If you have a damp crawlspace in a humid climate, it might be better to seal the ground with plastic and gravel, and use a dehumidifier to control moisture levels. If humidity is not extreme and the air tends to dry things out, I recommend passive vents in the foundation and floor insulation between the joists.