Role of cavalry in ww1 was huge. Although a cavalry charge was no longer a viable military tactic, horses were still invaluable as a way of transporting materials to the front. Military vehicles, as with any mechanised vehicles of the time, were relatively new inventions and prone to problems. Horses, along with mules, were reliable forms of transport and compared to a lorry needed little upkeep.
Such was the use of horses on the Western Front, that over 8 million died on all sides fighting in the war. Two and a half million horses were treated in veterinary hospitals with about two million being sufficiently cured that they could return to duty.
Horse pulling gun
Covered wagon pulled by two horses