[2019] What was the reason for British Cruisers to have torpedoes?

Having a ship, a vessel of "Diplomacy of force" as per British cabinet of the timeframe, be armed to the teeth with various offensive capability rather than for a specific target does make sense for the time period. The US response for the cruisers, going AA heavy, was in direct response to their experience in the Pacific. The Royal Navy had more than a few centuries of "Gunboat diplomacy" as well as facing a wide variety of challenges. to learn that having a bag of varying tools was to its benefit.

And the British Mark IV and Mark IX were good torps for there era and need.