It all works together. Yes of course it helps to get guys off the field if they test positive. Yet the nature of the incubation period means there is almost no doubt players have played while being contagious.To clarify - Covid has not been spread from team to team in games because testing has kept infected players off the field - so far. But all it takes is a false negative test to change that.
Regardless, stay in your "bubble" and you've got a very good chance, I'd say 99%, of playing your games and seasons. Venture out of that, and you put it all at risk.