Cruisers were built for long range patrol in big oceans in the pre-war world. Their large size gave them the required range and also meant they could get close to their design speed in rough weather. They carried aircraft, allowing them to cover more ocean than a destroyer flotilla. In the days before compact centimetric radar a cruiser also had huge advantages in terms of fire control.
Agreed that the "10000 ton" cruiser was more a result of treaty limits than actual usefulness. A main armament with no AA capability plus a secondary armament divided into port and starboard batteries was not efficient.
In actual wartime engagements destroyers gave much more bang for your buck overall but still fared very badly up against "proper" cruisers in daylight. See
https://en.wikipedia.org/wiki/Battle..._Bay_of_Biscay for example.
A modern frigate is the size of a small ww2 cruiser, whilst a modern destroyer is pushing 10000 tons displacement again. They also have oceanic range. Crew sizes have come down thanks to automation of armament but are still above a WW2 destroyer.