So, thinking about surveillance in e.g. schools.

If your choice is to test all the students monthly -- e.g. 3% of them per day -- with a molecular test that has a couple day turnaround... This works pretty much only well enough to detect rampant, uncontrolled spread, and will not break a transmission chain very often.

Or to test all the students weekly -- 14% per day -- with an antigen test that returns results in 30 minutes... Even if there's some false negatives, you'll still interrupt many transmission chains and effectively lower Rt.