During the War against the GLA, a GLA hole would be left behind if a GLA building was destroyed. It was very helpful to survive one-blast superweapons like nuclear missiles or MOABs (but not the particle cannon). From within the hole, a worker would emerge to rebuild the building in question, at no cost to the owner. The worker who performed this task disappeared back into the hole after construction.
Enemy commanders had to be careful to eliminate the GLA hole each time they destroyed a GLA structure, or the hard work of their troops would be undone. However, holes are rather easily destroyed, not delaying a thorough attacker much.
The GLA hole also has an unused animation of the door opening, so the worker can go through and rebuild the building.