Can someone please explain why testing (of non-NHS staff) is so important

[Celeriacacaca]

I get why it's important to test NHS staff so that they can work, rather than isolate unnecessarily, but can someone explain to me why widespread testing, which we're not doing, is so vital in slowing the spread of the virus please?
How does it work - I hear talk of tracing contacts etc? What happens then? Someone may not have Covid when tested but contract it a few days later, so how does that help things?

I think it's more about knowing who has had it.
If you've had it you are 'likely' to be immune and can carry on knowing you probably won't catch it again and you don't have it so you can't pass it on.
Also if you can detect you have it quickly then you can isolate and not pass it on.
I'm sure much more knowledgeable people will be along to explain far better and I'm interested to know as well.

Thanks @hellsbellsmelons - yes, the "who's had it" makes sense too, although I thought there was doubt over whether you are immune once you've had it? So many questions which I guess is the problem with the virus as there's so much unknown.

It’s just another method to try and slow the spread. By testing people who are symptomatic, they either get a negative which they can carry on working (if essential) etc or they get a positive in which case they are far more likely to follow the guidelines and stay indoors. Then they can start tracing anyone that person has been in contact with, not to test, but to inform that they have been in contact with a positive case and they need to isolate for 14 days. Otherwise they would carry on potentially spreading in the community.