Your shield can always protect you, if you so choose. However, it can be useful for more than just defending. Shields are relatively large, heavy, and hard to counter because you are attacking and defending at the same time. So why not go on and hit your opponent with it? As shields are less suitable for that than weapons, they do less damage but sometimes cause Standard Status Effects, stunning is the most popular result. Using shields that way is a staple characteristic of shield-wielding type of Stone Wall. Also, shields can have spikes on them (often a single one in the center).