A good Militaristic Policy to have if you are an expansionist bully who has serious problems with the other leaders and can't trade for additional luxury resources. Chances are you will then have enough units to station in your cities, at least when not at war. The catch is that you will need it mostly exactly when at war, due to War Weariness pulling down your general Amenities count; so, you should separate some units to remain as garrisons and make use of Retainers, while your main army is out conquering.
If you're not a militaristic civilization, you may not need this Policy at all. And chances are also that you won't go with Monarchy, which is the Government with big amount (3) of Military slots, but with another government type - this means that there are other Policies which may be more useful than Retainers. (Conscription remains always useful, and a possible second slot may be used on a rotational basis for a building-oriented Policy). Still, if your strategy doesn't call for anything else, and you have spare slots, you may use Retainers to boost your cities' general performance. Remember to keep units in each one, though!
Civilopedia Entry Edit
In the counterfeit feudalism that prevailed in the late Middle Ages, a society organized along lines of personal service being a “retainer” was the best method to obtain favor in the form of offices, land grants, gifts, and honors from those higher up the nobility chain. In return, the gentry helped fight their lord’s wars or put down rebellions. Not only did the gentry have to fight for the king themselves, but they were required to supply an agreed upon number of lesser nobles and knights as well. These personal retainers were known collectively as the “affinity” of the lord, akin to the celebrity “posses” of today … but more reliable.