Just like how damage dealers have openers to bring out huge dps numbers, tanks are doing the same but with trying to maximize aggro generation at pull in order to take massive lead with it. Then they can switch off their tanking stance and dps. This is bit hard to accomplish because they cannot see 'how much' aggro they are generating. It will differ from party to another, some tanks might to keep their tanking stance longer than others.

This is about the only advantage I can find.