The 'Large Light Cruisers'.
The 'Light Battlecruisers'
'Fisher's Follies'
'Curious, Outrageous and Spurious'

But why build ships with 4 x 15-inch guns, a measley 3" armour belt, and 32 knots speed?

(Furious was a little different)

(1/9)

Traditional light cruisers were too slow in a seaway compared to the battle cruisers to carry out their usual scouting / screening job. The large light cruisers had the size, seakeeping and speed to do so. Giving them 15-inch guns as well had a couple of justifications.

(2/9)

First, against enemy light forces 1 or 2 15-inch hits would likely prove decisive. This would enable them to rapidly overwhelm enemy scouting forces. If confronted by enemy battlecruisers they could use their superior speed to fall back on their own 'heavy' battlecruisers.

(3/9)

Decimating the enemy light forces would enable British light forces to make torpedo attacks on the enemy battlefleet, freeing them from their primary concern of defending the British capital ships from such attacks.

(4/9)

Secondly, while inadequately armoured to stand in the line, with their high speed they didn't need to be in the line to contribute. They could stand off on the disengaged side of the British battlecruisers, somewhat shielded from fire, and engage at relatively long range.

(5/9)

They could concentrate on the van or rear of an enemy, and essentially support the battle cruiser force. They had the speed to get out of trouble if required, but could add their fire to already engaged ship to bring about a quicker result.

(6/9)

Thirdly, a significant concern was German auxillary merchant raiders. These were possibly large and fast ships, and a 15-inch gun would be far better at disabling these quickly than a typical cruiser gun.

(7/9)

So all in all the large light cruisers were a potent scouting force, able to rapidly overwhelm enemy screening forces, before falling back on the 'heavies'. They could then contribute to that engagement as well. They could also do trade protection.

In theory, anyway.

(8/9)

Ultimately, that they got turned into carriers was probably for the best!

(9/9)

I await @simonharley 's corrections!