Sheilds were especially useful to a character that was wearing little or no armor. Wonder Woman's Bracelets of Submission were made of a bulletproof metal that was also very resistant to energy, and as such, they too acted as shields. Still they didn't cover her entire body and she had to be quite careful when in combat, especially against opponents with guns.
As a shield was usually used for defensive purposes, one holding the shield generally carried a sword or a gun as well, so that they could fight back. That isn't to say that a shield couldn't also be used as a weapon though, since that has been known to happen as well.