If the temperature drops below 32º F for too long, you run the risk of ending up with frozen pipes. Frozen pipes are a really really bad thing. First of all, when water freezes, it expands. The expanding water can break pipes, cause leaks, and cause thousands in damages to you home and plumbing systems.
One way to prevent frozen pipes is to use a hot water recirculator to constantly move water through the system. As we all know, moving water does not freeze very easily. Think about flowing rivers in the mountains covered in snow.