It's an antibiotic. Severe patients will be dealing with secondary (bacterial) infections, so it makes sense to use both. But on its own, I expect it would have little effect.
It’s already commonly used as both treatment and profylactic (in healthy risk group patients) for viral pneumonia. Why it works for I think is still not entirely clear.
Being an antibiotic is also convenient because many times you would also get antibiotics to prevent a secondary infection from bacteria.
It’s not exactly a miracle cure (like an antibiotic is against a bacterial pneumonia), but doctors have few tools when it comes to viral pneumonia, so even something that helps a little is welcome.
