Riding an animal is not difficult to do, but doing it without getting hurt or very tired is an art. This skill is required not to stay on your horse for the journey, but to manage your body properly in order to not get hurt or worn out.
Failing the check will add additional Wounds due to exhaustion for the journey for both you and the animal you are riding on. If you do combat from the back of an animal, you need to first succeed a riding check before you can attack. A failed roll means that you do not have proper control over your animal and you can't get to the proper position to strike. The same holds for ranged combat from the back of an animal.