Horses' legs, especially the tendons and the fetlock joint, are very sensitive to injury. On the one hand, these injuries can come from brushing the leg with the hoof or from hitting things like obstacle poles externally. But the tendons can also be injured by excessive strain. Horse boots or even bandages are only of limited help against such strain. In this respect, they are mainly useful for convalescents after such injuries.
Tendon boots offer the best protection against the horse's own injuries and against other external mechanical impacts. However, you must make sure that the horse boots are not too loose. If they slip down the leg, they can actually become a danger for the horse. On the other hand, horse boots must not be too tight, so that they do not cut off the blood supply to the leg.