Like has been said, shield body armor is just like armor for you, it reduces damage that comes into contact with the shield. The higher the armour rating on the shield, the less damage it takes.
Usually its a trade off since high armoured shields have low hp and high hp shields have low armour. But the only time they help you with their armour rating is when they're on your back and you get hit by ranged in the exact spot your shield is then it helps reduce the damage. But that's useless since you'll never have a shield on your back.. or you shouldn't anyway.