It helps to flatten the "curve", and thereby lessens the impact on overwhelmed ERs/ICUs...
And saves lives until treatments, vaccines, or heard immunity kicks in.
You would have to quarantine for months to flatten the curve. That will not be feasible. Most people will only tolerate three weeks before they ignore quarantine.
Vaccines will take 18 months.
Treatments ?
Herd immunity only if the young go back to work.