LogFAQs > #961221713

It's the whole "flatten the curve" thing that bounced around so much in the early days. By imposing lockdown measures to reduce case rates, you cause it to take longer for the whole population to gain immunity by being infected, but you prevent the total case load from getting higher than the health care system can handle. In practice, natural immunity doesn't seem to be lasting long enough for "the whole population will eventually reach herd immunity on their own" to be a valid philosophy, but it's still beneficial to keep case numbers low enough to avoid overwhelming hospitals while we wait for vaccination rates to get high enough to improve the situation (at least, it would have been, if people didn't suck).

Duckbear's explanation isn't exactly helpful, but he's quite correct.