The rider combines the technicality of approaching the obstacle with the height to be crossed.
We say that we jump bars when we practice show jumping. Indeed, for this discipline, no fixed obstacles, only bars and bases which fall if the horse touches them.
Conversely, when we say that we jump from the fixed, it reminds of jumps on the cross. The obstacles are hard and still. If the horse touches, it will hurt itself, or even fall with its rider. It is a dangerous discipline which requires special protections.
It does not matter whether the horse is going to jump bars or fixed, whatever the level, the first thing that we protect are the tendons of the cannons, and the balls. Indeed, we protect these parts of the body because by jumping or landing, the horse can touch itself with the hind hooves.
The tendons, which are very poorly vascularized, repair themselves poorly: they must be warmed up and properly protected. The fetlock balls are the round joints that sit above the hooves. They are used to finalize the movement, as important as the thumb of a hand for gripping an object.
There are also protections used for dressage, or specific disciplines such as reining in western riding. These disciplines place great demands on the tendons of horses: we can protect, support and warm them according to the technical nature of the protections.
You can also use protections when you go for a ride, to limit the risk of impact of the hooves on the limbs, if the horse sometimes has disordered movements. For young horses, or those who have problems with their plumb, the timid ones who make gaps: there are many reasons for using gaiters outside of the jump.