Moisture finds its way into your crawlspace because of negative air pressure existing as a result of the way air moves through your home. This moisture can be in "bulk form" which is actual water on the ground, or water vapor and high relative humidity (RH) which is in the air. Unless you have serious flooding issues in your crawlspace, putting pressure on your foundation walls, water vapor and RH are the real issue here. As that vapor reaches cooler areas in the crawlspace by the movement of air, it reaches a dew-point and condenses, leaving building materials damp with a possibilty of growing mildew or mold.
Even with a good vapor barrier covering the dirt floor of your crawlspace, moisture is still able to enter through the crawlspace vents. The old adage of keeping the vents open came from the idea that the crawlspace is wet, you need to vent it in order to dry it out. However, years of studying crawlspaces has shown that venting crawlspaces actually increases the humidity.
It works like this: Even in the summertime, your crawlspace is significantly cooler than the ambient air outside. In thermodynamics, hot moves to cold, meaning that ambient warm air outside the crawlspace wants to get in because there is a temperature difference. As that warm and humid air goes down into the crawlspace, it reaches its dew-point on the colder surfaces, dumping its moisture into the crawlspace and continuing on out, or more likely, into your house. This is often what you see when you get in a crawlspace and you notice a rainforest effect, where the HVAC system, the floor joists, and the insulation are dripping wet.