The theory is that for most people under 65, the measures we are taking to suppress COVID are worse than the disease. So, scarce resources, like vaccines, should be used to protect the small part of the population that is likely to die. Don’t waste doses on those who have laready had the virus and recovered, or on people under 65 without complications conditions until the vulnerable population is safe.
Once they are protected, open the economy, let people take the precautions they choose, and let herd immunity happen through the combination of continued vaccination and the recovery of healthy, young people who aren’t at much risk.
COVID is only an emergency, justifying closing down the economy, because people die. Stop the dying and the emergency is over.