In vanilla Civilization VI and Rise and Fall, this policy isn't all that useful most of the time - by the time it becomes available, you should have plenty of Encampments to train units without a second copy of strategic resources, and it does no good unless you have at least one copy of a resource anyway.
In Gathering Storm, this policy can be helpful if you're pursuing a Science or Domination Victory: it will aid you with the former by providing you with Aluminum for Lagrange Laser Stations, and the latter by providing you with the resources needed to upgrade your army, navy, and air force.
Civilopedia entry Edit
The “efficient and effective development of an organization’s resources when they are needed” … resource management. As corporations grew larger and evolved into multi-national conglomerates it became vital for those wishing to survive to insure that resources were never over-allocated to projects. Perhaps the best method of doing this is “resources leveling” – financial, inventory, human, production output, and even information – avoiding both excess inventories and sudden shortages. The trick is to invest in resources as “stored capabilities” and then “unleash” these as necessary and practical. The spread of resource management principles have allowed corporations to engage in some immense, both in terms of size and duration, yet profitable projects since the 1950s.