?

It's a shield. It can still block.

And if you have the right perks, you can mitigate some magic, and it makes a fine weapon in its' own right.

This here and it can also deflect arrrows its a great weapon to bash and stagger foes too, it opens up a different avenue to play and great for roleplaying value.

If you don't use Shield, that hand might very well be empty. When you're casting a spell, attacking with a weapon immobilizes you for 2 seconds.

The benefits have been noted previously.

You're not going to hit the cap for a while, so a shield is useful for a large part of the game.

Not forgetting shields can be enchanted, or double enchanted, for additional benefits

Block stacks on top of armor and it is also capped at 85%. Combined with a high armor, it will reduce the damage to just a scratch. For instance a 500 point attack will drop to 75 from armor. With maxed block it will drop down to under 15. Pretty nice, especially if you play on master.

As mentioned, you can deflect arrows and reduce magic damage taken by half with a shield. With Block Runner and Shield Charge, you can become a battering ram. Shields are also more effective at bashing than weapons.

Combine block runner with the Slow Time shout for added fun.

Oooh yes...slow time with the 'slow time block' effect allows you to slaughter a bunch of enemies before the first one hits the ground.

Shields = A1+ fighting tool.