The Defense Facility is used for to allow the building of defensive structures in the Defense Area of your base and aiding in the repairing of those structures and vehicles following an attack.
Following an attack the defenses can be automatically repaired by a maximum of 70% for those units that are at or a lower level than that of the Defense Facility.
The Defense Facility is a Unique Building and therefore requires 1 clear plot spacing from other unique buildings.