Splint boots can help protect or possibly minimize an injury if your horse takes a bad step, stumble or whatever. I wish I had been using splint boots on Caisson the last ride we were on. He ended the ride with a cracked splint bone, and associated trauma to the tendons and suspensory. I'm not sure exactly when he did it - could have been a couple of places, early in the ride. (I suspect the near-bath incident with the water trough.) I think the damage could have been minimized if he had been wearing protective boots. I guarantee you, he doesn't leave his corral without his splint boots these days.
Also, why do so may riders put leg protection on their horses? I would think they might rub or fall off & cause problems? Do horses really need all this protection?????