I'm not a nurse or a medical professional but I think it's to cover ones basis and prevent any complications from encountering a antibiotic resistant strain in the patient. Using a single antiobiotic might not completely get rid of the infection/cure the patient. It might leave a weak remainder population of the invading infection. The second antibiotic is used to mop up after the first antibiotic treatment.
A deliberate overkill combination of antibiotics in order to give a one-two punch against the infection.