If I was in a Governor's or Mayor's shoes right now and I know there aren't enough resources to test everyone, I'd want some data like that to work with to make policy decisions.
While you are in the process of opening things back up, you keep a close eye on your daily/weekly representative sample numbers to see if anything needs to be dialed back. It gives you much closer to immediate feedback.
I suppose the immediate problem is getting a hold of a representative sample of people willing to be tested, but that feels solvable. I'm sure I'm not the first one to think of it, just curious why it isn't being done, as it feels immensely useful (maybe this is just the 538 political nerd in me wanting virus polls?).