
DonDuckie |

If you require a free hand for something else, like a wand or potion.
If you have a +1 bashing spiked heavy shield and improved shield bash(high damage+shield bonus to AC).
if you have shield slam(even if you don't use TWF)
or shield master.
weapon focus and weapon specialization with shield.
if you're fighting skeletons with sword and shield.
if you carry two shields - you can bash and defend(supposedly).
the phalanx soldier archetype can use a reach polearm or spear in one hand a still threaten adjacent with the shield.
many more reasons...