Right - this is just a way of helping to prioritizing who gets tested. Given that testing kits will always be a limited resource (you can't simply test everyone

Right - this is just a way of helping to prioritizing who gets tested. Given that testing kits will always be a limited resource (you can't simply test everyone, everyday) it makes a lot of sense to find sub-populations who are more likely to be positive. That doesn't mean you don't test anyone else (eg. those with symptoms, those in sensitive jobs) - it just lets you use a certain percentage of your testing capacity one those people who have been in proximity with confirmed cases. As you say, it's not a silver bullet but in combination with a slew of other approaches can help reduce the rate of transmission rate.