Antibiotics are useful for fighting bacterial infections, but they are not a medication that should be prescribed for any and every illness as they are completely ineffective against a lot of common illnesses. Antibiotics should not be prescribed for any illnesses caused by a fungus or virus. For example, athlete's foot is a fungal infection, so antibiotics will not clear up the ailment. Similarly, the common cold and flu are both caused by viruses, but this is something that I feel the general public is not often aware of. Anecdotally, I have coworkers every year that get a cold and make a doctor's appointment in hopes of receiving some kind of antibiotic to help speed their recovery.
Antibiotics can be prescribed when a person has a bacterial infection, and the person's body is not doing an adequate job of fighting the bacteria. I myself came down with keratitis in February. Bacteria somehow got into my eye and began destroying my cornea. Within 24 hours of realizing my eye was red and hurt, I couldn't open it. I saw the doctor later that day, and the report was had I waited much longer I was risking permanent vision changes and/or blindness. I was given 2 separate antibiotics to clear the infection, one oral and one eye drop. Five days later, my vision was back to normal. Another instance in which antibiotics absolutely should be prescribed would be in the case of a ruptured appendix. Far too much infection is spreading too quickly for the body to fight it all. Antibiotics are needed to save the person's life.