-If the person gets sicker later, you don't have to wait on another test to know the diagnosis and start treatment.
-Second is from a contact tracing perspective and isolation perspective. If this person is positive, it is that much more important that they limit contact with people.
-Third is if you are positive you can donate your plasma which *may help others (good randomized controlled trials for convalescent plasma are still pending).
-If you are positive you make not need the vaccine later, it also could reduce your risk potentially for being infected later, less stress.
-Also helps population wide to know total number of infected to get an idea of rate of spread, herd immunity, other metrics.