The wood beams and joists in your crawlspace are the foundation upon which your house is built on. For obvious reasons, it is important to keep them in good shape to preserve the structural integrity of the house. Wood decay will occur when the wood gets wet or has absorbed a high amount of moisture. Moist wood is also a primary food source for termites; something you definitely do not want to attract toward your home.
Wood rot can be prevented as long as the wood has the ability to dry out. This will happen naturally in colder temperatures and lower humidity levels. In peak summer months, the wood may not have a chance to dry out with the high temperature and humidity levels.
Generally, wood will be susceptible to decay when wood moisture levels are in the 20% or greater range for an extended period of time.
Once the house has wood rot, any moisture control system will only stabilize the situation. Any damaged wood will need to be properly replaced. The moisture control system is then important as a preventative tool to keep the situation from repeating itself.