If croup with woughping cough in infants is a result of viral infection then why doctors prescribe antibiotics ? is it for just in case scenerio?

Empiric treatment: Often doctors don't know what the cause of a cough like illness is, unless they have done specific testing for it. Sometimes there are clues that make a clinical diagnosis easy, but it's always good to have confirmation with a lab test-then the best therapy can be prescribed. You're right-antibiotics don't work for viruses, but if the doctor is trying to cover all the bases, you might get an Rx.