Why warships retain main guns aft

What is the utility of mounting main guns on the rear half of a warship?

They add redundancy, cover the rear quarters, and allow the ship to keep firing while it withdraws.

Aft main guns are not dead weight simply because a ship expects to fight forward. They provide redundancy if other arcs or turrets are unavailable and cover an enemy approaching from the rear quarters. Most importantly, they let a ship fire while withdrawing instead of turning back toward its pursuer. Renown and Repulse show the design balance: four guns forward support pursuit, while two aft discourage an enemy during retreat. The arrangement turns a preferred offensive geometry into a ship that can still protect itself when the tactical situation reverses.