What role was the 5.5-inch gun meant to fill?

What kind of target was the 5.5-inch naval gun intended to fight?

It was meant to combine enough power against cruisers with enough firing speed against destroyers, making it an attractive battlecruiser weapon.

The 5.5-inch gun was intended as a compromise between 5-inch and 6-inch weapons. It needed enough hitting power to engage cruisers while firing quickly enough to deal with destroyers. Those requirements made it attractive for a battlecruiser.