When houses have full basements, the occupants tend to become very aware when water seeps into the lower levels. That is not always the case with crawlspaces. In fact, crawl spaces are rarely entered and it is not at all uncommon for owners to not know their condition; to include knowledge of water seepage. This image is a classic case of what the consequences can arise when things are out of sight and hence, out of mind. The floor joist seen here is extensively rotted and the insulation material installed between floor joists has fallen out of place. The wood rot is so advanced that extensive and expensive structural repairs are needed.
The high likelihood is that water seeped into the crawl space because of an inadequate roof and yard runoff controls. The unfortunate reality is that professional home inspectors see things like this on a routine basis. It is very important that a crawl space is thoroughly inspected prior to purchasing a house. Water is the primary cause of damage to properties and when a water problem exists for a long time, repairs are likely to be extensive. A wet soil condition around and under a house also creates an attractive environment for wood borers. This is one reason why so many professional home inspectors routinely recommend roof and yard runoff controls to potential home buyers.