Protection pretty much explains itself doesn't it? If you look through the protection tree you'll notice most of the points you can spend benefit spells that gain lots of threat. It doesn't matter about the damage you put out it matters about how much threaten the enemy.
You'll notice the only tanks you find will be,
Protection (Warrior)
Protection (Paladin)
Feral ( Druid )
Blood (Death night)
All because they are the classes that are made to tank with the spells they get/spec.
The reason you have a shield is because as a tank your job is the get all of the enemies to attack you, now if you had two weapons like you can have as fury you would be taking a lot more damage. When you have a shield on you get heaps more armor and a chance to block enemy attacks. ( IT basically makes the healers job a lot easier)
Sorry i didn't exactly answer you questions in order but it covers the lot
Happy Gaming.