A very common problem I find in my area of the rainy state is excessive moisture in the crawl space.  More specifically, standing water.  Standing water can lead to problems from the "M" word to structural damage caused by wood destroying organisms.  The usual suspect is a negative grade towards the home that simply allows runoff to accumulate in the crawl space.  Given the scenario, one could either re-grade or install a sump system.

I recently inspected a home that indeed had a sump system installed, which wasn't working.  

Like a thourough inspector should, I proceeded to investigate where the water was coming from and followed the wet vapor barrier to a corner of the foundation wall that was wet and showed signs of an active moisture intrusion.  Well, what did I find directly above the the wet area?  A leaking P.R.V. from the hot water tank.  Apparently, whoever noticed the accumulation of water in the crawl space installed a sump system without verifying the source of the problem.

Tacoma Home Inspector