Why Japan built torpedo cruisers

What was the idea behind converting Kitakami and Oi into torpedo cruisers?

They were intended to mass long-range torpedoes for night attacks, but the concept relied on an enemy following a predictable course and was vulnerable to zigzagging and radar.

The converted cruisers were intended to overwhelm an enemy battle line with a massed long-range night torpedo salvo. The concept worked best against an opponent maintaining a predictable course, as some US formations did in early Pacific battles. Royal Navy officers had absorbed habitual zigzagging after the loss of Aboukir, Cressy, and Hogue, so British maneuvering would disrupt the firing solution before technical comparisons of guns or torpedoes became decisive. Radar, volume of fire, and stable British designs would further reduce the Japanese advantage once the long-range salvo failed to land as planned.