I don't know if I'm missing something, but for me, testing seems to be as useful as the MOT test for cars. It only shows the status on the day the test is carried out.
Surely unless you are tested every day, then they just lull you into a false sense of security. As has been pointed out further up the thread, as soon as you walk out of the testing centre, you could pick up the virus. Especially if someone who is also at/around the test centre, is positive, and their droplets are still in the air, even if they are not right next to you.