As you would think, the rope has a coating that repels water. Not only does it reduce the amount of moisture absorbed and thus weight, but it also makes the rope slicker. This may help to reduce drag in some climbing environments. Care and store your ropes properly. You really need to baby your climbing gear.
Dirt can damage ropes in a similar way it damages rugs.
The dirt gets worked into the rope and begins to cut the threads. If your rope gets dirty you can carefully wash it and then dry it properly. Follow the manufacturer's directions for washing. You should never store or dry your ropes in direct sun. The sun brakes down the material over time.