When chickens, and birds in general, come down with any kind of respiratory disease it very often and very easily morphs right into pneumonia and secondary bacterial infections. Doesn't matter if the disease is viral or bacterial these complications are very common, that is what very often kills the birds rather then the disease itself. So if one chooses to treat the birds rather then cull antibiotic's will help prevent those complications while the bird recovers. Just be aware that even after the bird is better it will be a carrier of whatever it had with the potential to infect other birds in your flock and may have recurring outbreaks of the disease as well. Some diseases are worse in that regard then others.