What happens if you don't treat kennel cough?

We're fostering a dog. He went out of town last week to an adoption day event, but instead of getting a new family, he got kennel cough. Poor baby. He was taken to the vet and has a bagful of meds to take for two weeks. But we didn't get the chance to ask the vet all about the disease. Why is it considered to be so serious that we vaccinate for it? What would happen to an untreated dog?