Before the development of more mobile Command Centers, Strongholds served multiple capacities ranging from habitation, supply storage, construction facilities and defense. During the Guild Wars, these structures were repurposed by government officials as secret heavy bunkers against raiders and rebels. With several stories built underground coupled with multiple layers of Neosteel walls, users were able to hold out for months until: a) backup arrived, b) raiders leave with whatever they could find, or c) rebels finally penetrate the structure and haul out the officials. To avoid the probability of capture, some Strongholds have teleportation pads to transport personnel to a more secure area.
After the Great and Brood Wars, many fringe worlders repurposed Strongholds as a communal "bomb" shelter against any more alien invasions. Upgraded with whatever technology could acquire, Stronghold defenses are roughly equal to a military grade Planetary Fortress including multiple gun and flame traps.
As for their construction facilities, Strongholds are only capable of producing civilian and obsolete military vehicles.
Should Strongholds be captured, the invading force could easily secure store housed containing tens of thousands of raw materials and supplies.