Side passing is good exercise for the horse helping with building strength and muscles. If you will look at dressage you will see different ways of improving your horses strength through different moves. I believe in ancient times side passing may have been a movement in war to protect horse and rider. It is really beautiful to see a horse who is well trained and a rider who can ride a well trained horse. But be careful if you are training your horse and ask for a professional's help and advice. Good Luck and best rides!