Despite the nature of the ability, with patch 5.4.0 Shield Wall no longer requires a shield to be equipped. If the warrior does not have a shield equipped, a visual of an equipped shield will be displayed.
Shield wall is one of the most powerful forms of damage reduction in the game, and is very effective at keeping the warrior alive. In addition, shield wall does not drop aggro, so it allows a warrior to continue tanking.
Shield Wall is a great panic button, and is useful when you just have to survive a little bit longer. Examples include an OOM healer, just a few adds too many, an enraged end-instance/Raid Boss, too many stacks of a debuff that reduces healing received, or a battleground flag run.