Great officers professionally rode on horseback. The horse was more reliable and faster than mechanized vehicles in adverse conditions! But the most important role was in supplying the frontline so the the men could stay engaged in battle around the clock. These animals, on whom lives depended- They were even known to ferociously charge into the fiend's rank, breathing fire through their nostrils and to frighten the life out of them.
The question is not why to add the horses.. the question is why they haven't been in the game from the start.