You use fetlock boots to protect the horse's hind legs against over reaching. The hard shell of the fetlock boots protects the fetlock on the inside. If you use a hind boot, the fetlock will be protected on both sides. Because this protector fits all the way around the leg, it will not shift if your horse is over reaching a lot.
Do you also use tendon boots? Then you can mix and match a set with BR fetlock boots or you can opt for a complete set.