The screenshot you post shows exactly why it's there, but it has some deep medical speak. It's a multi-drug cocktail. These have been very successful and are the reason people with AIDS no longer have a death sentence.

Ritonavir is a protease inhibitor. A protease is a something that breaks down proteins. So the protease inhibitor is a drug which helps the other drug, in this case, nirmatrelvir be effective for longer which means you can use less of it. With AIDS when they first started, the drugs had bad side effects, keeping the doses low was helpful

PAXLOVID is comprised of nirmatrelvir, a SARS-CoV-2 main protease (Mpro: also referred to as 3CLpro or nsp5 protease) inhibitor, co-packaged with **ritonavir, an HIV-1 protease inhibitor and CYP3A inhibitor. Ritonavir, which has no activity against SARS-CoV-2 on its own, is included to inhibit the CYP3A-mediated metabolism of nirmatrelvir and consequently increase nirmatrelvir plasma concentrations to levels anticipated to inhibit SARS-CoV-2 replication. **PAXLOVID is not approved for any use, including for use for the treatment of COVID-19.