Any horse, no matter which equestrian discipline he will perform in, should be trained according to the classical principles of the Training Scale. These are principles which are vital to develop the horse in the correct way. Therefore the basic work with a show jumper is no different. Plus, I always tell my show-jumping riders that to be able to ride leg yielding and shoulder-in (especially shoulder-fore in canter) improves the engagement of the hind legs of the horse, makes the "engine" stronger, and gives the rider more feel and control of all "four corners" of the horse. Why to ride sideways? Because it improves the ability of the rider to ride the horse straight!