For knowing where to focus limited testing resources, for one thing. And potentially for locating asymptomatic carriers.
Explain your thinking. If you test a building and it tests negative, does that mean people in that building don't need to get tested? If it tests positive, then people should get tested? Why or why not?
Again, I repeat, it's imperfect.
You're responding to your assumption that you think I'm criticizing what you said for being 'imperfect.' I don't care about perfection in the least. I am just responding to things I read on line trying to think them through publicly.
Right now, the perfect is the enemy of the good. The medical community needs to find vectors. This is one admittedly imperfect means of doing so.
How is "the perfect the enemy of the good?" Why does the medical community need to find vectors? Do they think that they can really trace every person who has come into contact with someone who has tested positive?
Social-distancing and minimizing going out reduce the opportunities for the virus to be transmitted, but you could still get it from touching something at a supermarket, or touching your mail, or from food you eat, etc.
The social-distancing and other precautions are probably reducing the total quantities of contact situations by huge amounts, but there are still huge quantity of contact situations through which the virus can be transmitted.
Let's say before the lockdowns and social-distancing there were a billion potential contact-transmission interactions per person and now there are a million. A million is a huge improvement over a billion, but it's still too many to keep track of.
Some employees of various businesses are being reported in the news so people who shop at that store can go get tested if they want, but of course you never know if it's fake news geared toward getting customers of that store to pay for testing. There are always business interests that skew/spin things in the interest of making money.