Imagine that you have 10,000 threat on a single enemy. Now your mage buddy gets a lucky string of crits and hits the enemy really really hard, and now has 30,000 threat.
If you use taunt in this situation, the enemy is forced to attack you, and you now also have 30,000 threat on that enemy. This ensures that the enemy continues attacking you after they are no longer forced to hit you, unless your mage buddy were to continue generating threat faster than you are.
If you use mocking blow, the enemy is forced to attack you, but you still only have 10,000 threat, and once mocking blow has worn off, the enemy will go back to attacking your mage buddy.
If you use mocking blow with the barbaric insults glyph, then the enemy is forced to attack you, AND you now have 30,000 threat on that enemy.