We know we can't stop it. But the goal is to level out the rate of infection. If everybody (not everybody but hopefully you know what I mean) gets infected within a small time frame, resources are beyond overloaded and doctors will have to decide who gets resources and who doesn't (and potentially dies). It's a multiple casualty incident at every hospital at basically the same time.
If we can manipulate social behavior (for example, limit large gatherings of people), we can better prevent resources being overloaded (meaning, there are enough ventilators, nurses, medications, oxygen tanks, beds) and better provide help for those who need it.