Sealing up your crawlspace creates a controlled environment similar to how to keep the inside of your home. A sealed crawlspace will prevent any moisture coming in through the vents and the dehumidifier pulls any moisture from the air to keep your entire crawlspace regulated.
The ground carries a lot of moisture. Sealing the crawlspace and installing a liner to cover the ground will reduce the moisture levels in your crawlspace. This will prevent a lot of problems like wood rot, structural issues, mold growth and sagging/hanging insulation.
We would design a drain system that goes around the perimeter of basement that would drain to a sump pump. Next, we would inspect outside of the home to pinpoint any areas of intrusion and develop a plan. Some exterior options would be installing down spout extensions on the gutters to push the water away from the home and foundation. We would also inspect the gutters on the home itself and see if that could be the root of the problem. There are a variety of ways water could enter your home from exterior factors.
The spots could possibly be mold. In this situation, we would perform a mold test to figure out what species of mold it could be and then remediate the entire crawlspace.
A french drain system could be a great solution to this problem. It is designed to retrieve water that enters the crawlspace and divert it to a sump pump which pushes it out of the area.