Slam,
There's a couple of reasons that you might want to add runoff rather than runway. One is that you've got to be able to make a safe approach to the runway threshold. That might not be possible due to local obstacles.
And the other, more common reason is cost. A runoff area just needs to support the weight of an aircraft only in an emergency. Runways have to repeatedly withstand the shock load of hard landings and thus must be much thicker.