Why did horses wear armor in medieval armor?

The functional full suit of armor is an outer covering which helps in protecting against the external danger. So not only knights but also it is necessary for the horse, who is going on war, to wear some protection.

To protect them from injury, the same as the rider. When a horse and rider go charging into battle where others are swinging sWords, axes, maces, and trying to skewer each other with lances, some blows are sure to land on the horse. From the rider's standpoint, knowing he's dead if he falls on the ground, it is better to keep one's horse alive and mobile, which means keeping him uninjured.