This is another of the early Policies which accelerate military units' production time. It is usually much more useful than Maritime Industries (since making cavalry is always useful); however, note that you will only have real use for it if you truly intend (and need) to build cavalry units (this means the Horseman); and if you have Horses. Otherwise the only unit that benefits from it is the Heavy Chariot, and its Production cost isn't that great anyway. In such case you'll be better off using your Military Policy slot for something else.
The usual advise when it comes to production-oriented Policies is also applicable here: try to plan in advance when you will produce the units, then make a concentrated effort so that you can later remove the Policy and replace it with a more generally useful one, such as Conscription.
Historical Context Edit
Once warfare moved past the stage of small masses of men just having at it in a club-thumping melee, maneuvers gave the more nimble warriors a distinct advantage in battle. The concept of using rapid movements to keep the enemy at a disadvantage is almost as old as the idea of war itself. Tactical maneuvers are intended to be used on the battlefield, while “maneuver warfare” covers all that leads up to forcing an enemy to fight at a place or time they’d rather not. Both types came to prominence with the advent of chariots and cavalry … and their importance hasn’t diminished since.