The difference is in the quarantine: If I have to assume it's SARS2-CoV, I don't leave the house, my partner should stay at home as well because she's most likely also infected by now; also we need to get someone to bring us supplies some time the next week before ours run out. We just had that very drill a month ago due to a viral infection (medication only eased symptoms, all we had to sit it out and follow the usual hygienic/no-social-contacts rules not infect others).

Question is: What's the threshold? Can I trust external data or do I need to assume it's too imprecise?

Now if I knew that I'm most likely not infected (-> potential importers tested) I would still stay at home (in home office now anyway) and reduce social contact, but I could get supplies for family/friends who are quarantined. If I knew I wasn't infected prior to staying at home (-> everyone tested), and if I started developing symptoms, I would know that it's highly unlikely that I was infected and could thus reduce the load on the doctor/hospital (edit: I have "respiratory precondition", hence it seems I can't just say "well, I'm young and healthy, worst case I sleep it out").