As said upthread, testing isn't just for illness. In fact in my case, only 1 test has been done because I was feeling under the weather. The one positive test I have had was done because my wife had tested +ve two days earlier. So I was doing daily tests. - Because I didn't want to be mixing with other people and potentially passing on the infection, even if I was feeling OK. You know; social responsibility.
Other examples, illustrative not exhaustive:
Up to very recently, work asked for an LFT to be done before going in, to prevent infection spreading. In fact since this has been dropped, there are now almost daily emails about someone who had been in, has tested positive, extra cleaning, blah blah.
Tests are being done every time I, my wife, or my daughters, visit my Mum. - Vulnerable person.
Tests are done before I meet up with a friend in his mid 70s and I know his daughters do the same. My wife does the same with a friend of hers around the same age. And we also know they test before seeing us. - Social responsibility.