Once you know someone was infected, you need to go back and see where they've been up to that point. That's only possible if you're tracking everyone. Of cour

Once you know someone was infected, you need to go back and see where they've been up to that point. That's only possible if you're tracking everyone. Of course you don't need to keep the tracking data on everyone for more than ~14 days. Yes, there are other ways to do this tracking and contact tracing, but they are much more labor-intensive and imperfect. Governments can and should still do this, but once a certain percentage of the population becomes infected, it's no longer feasible. Also the more contacts that are missed through these tracking measures, the more potential avenues for spread. Hence the attractiveness of a system that would both be more comprehensive and require much more manual work.