Notice that the CA style shelter in place has an uptick at the end. Once you let people out it starts right where it left off. My assumption is that this thing is not going to be eradicated and is highly contagious, so the only way it will stop is once there are enough people vaccinated or previously infected so the exponential decays. Under those assumptions, slowing it serves two purposes. One is to prevent deaths due to overwhelmed treatment facilities. The other is to prevent it from just hitting everyone. Maybe only 20-60 percent of the population needs immunity to stop it, but if it runs rampant we could overshoot that. I'm sure there are other reasons to slow it, and other models, other assumptions, etc...