Before the war, they were considered necessary in an army. But from the development of artillery and machine guns, it was quickly realized that they maybe shouldn't, and were replaced by tanks at the same time. Horses during WWI were used mainly for reconnaissance and sending messengers, as well as pulling artillery, ambulances and supply wagons. The benefit of them was that they could travel better over muddy or rough terrain. WWI was kind of the turning point at which they stopped being in use, though they were used a bit in the beginning.