Water heater back up drain pans are designed to protect the floor of the structure in case of a leak from the water heater.  The water heater’s tank wil eventually rust through and begin to leak when it reaches approximately 8 to 12 years of age.   The drain line  from the back up pan should be routed to the exterior of the structure where it is visible to the home owner so if and when the water heater fails they can be alerted.