Actually COVID is one of the most used tests they do, at least around here.
But you can do things like drug use, cancer epidemiology (for some cancers), etc. as well - and that is incredibly helpful from a public health point of view.
Because it's just like with Covid - we can't get proper data from patient sided tests because we can't test everyone. And even if we could,not everyone would.
But everybody poops/pees. And guys like OP interpolate from that.