This policy allows military units to increase the Loyalty of cities in which they are garrisoned. It's very useful if you're trying for a Domination Victory and you need to assimilate conquered cities into your empire before they revolt.
Civilopedia entry Edit
Limitanei were the frontier garrison troops of the late Western Roman Empire and early Byzantine Empire, their name deriving from the same root as the word for “military border district”—limes. Over time these troops devolved into a locally recruited militia under arms for the Romans, and lost some of the prestige and professionalism of the other forces of the empire.