I guess it's less to do with negative tests which aren't that useful as you've already pointed out and more to do with positive tests. If someone knows they have the virus they can isolate and this will make a small dent in the number of people infected.Im aware of this but Ive never been for a test just like 60 million others in the UK. Unless everyone gets tested frequently, what's the point? Even if you get tested one day you can catch it the next so you'd be spreading it unknowingly.
Aside from the examples I gave above I dont see the purpose of testing.