Where a five-turret County-class cruiser would matter

Where would a five-turret, ten-gun County-class cruiser be more useful than the historical eight-gun ship?

In cruiser actions such as River Plate, although a standard County would already provide most of the needed advantage.

A ten-gun County offers a heavier broadside than the standard four-turret design. A River Plate-type cruiser action is the clearest place where that added firepower could help. Even a normal County would have been a major improvement over Exeter's six-gun battery. The fifth turret is therefore an advantage rather than a capability required to make the mission viable.