When Limes first becomes available, it's very useful for speeding up the erection of Ancient Walls around your cities. Given the level of aggression of the AI, especially on higher difficulties, you will need these, and maybe not only in your border cities. 100% Production bonus means it will take exactly half the usual time to build defenses, so use the opportunity to build these in as many cities as you can.
This Policy is especially useful if you go with Monarchy as your government, because of the additional 50% bonus to construction speed and Housing bonus that Walls confer to cities at a time when they're usually either close to or at their Housing limit. Georgian players can quickly build city walls and the Tsikhe throughout their empire with this policy.
Limites (the plural of limes) delineated the fortified frontiers of the Roman Empire, signified by fortified cities and military outposts. Hadrian’s Wall was often referred to as the Limes Britannicus; the far eastern edge facing the desert was the Limes Arabicus … the Limes Germanicus, Limes Alutanus, etc., etc. The military bases of the limites were designated as castra (fortresses of the legions) and castella (bases guarded by auxiliaries, usually local native infantry). Although the empire always looked to its defenses, the period between 40 and 280 AD was the highpoint of building limes. Indeed, Roman writers of the time presented the limites as a sort of sacred border beyond which civilized people did not go.