I think the preventive part is for the same reason it is a treatment...
It stops continued replication in an infected person and then reduces it...by the same dynamics, it prevents the spread of the bug if the person already has it in the system when the bug is introduced...”nip it in the bud” .
At a time when India is seeing a dip in its tally of new cases every day, the revised guidelines have also dropped drugs such as hydroxychloroquine, ivermectin, doxycycline, zinc and multivitamins, that were earlier prescribed by doctors to treat asymptomatic or mildly symptomatic Covid-19 patients.
In asymptomatic cases, the revised guidelines have said no medication is required. In the case of mildly symptomatic patients, self-monitoring for fever, breathlessness and oxygen saturation (SpO2) have been recommended.