The main advantage of antibiotics is that they improve and save lives. Antibiotics work to slow or stop the growth of bacteria, so they can cure or shorten a variety of bacterial diseases, including syphilis, gonorrhea, strep throat, diphtheria, scarlet fever, rheumatic fever, and Lyme disease, to name just a few. They can also stop or prevent infections caused by wounds, surgery, or poor dental care.
Antibiotics added to feed have been proven to cause baby chicks and turkeys to grow and gain weight faster. This practice is somewhat controversial because of the likelihood that it can lead to the evolution of antibiotic resistant strains of bacteria, which can be very dangerous.
One con is related to public perception. People often do not understand that antibiotics only treat bacterial infection and have no effect on viruses, so sometimes they will try to treat a viral disease like a cold with an antibiotic, and this not only won't help the cold, it can contribute to the problem of the development of antibiotic resistant bacteria.
Other cons are that using antibiotics can wipe out the friendly bacteria that we all have in and on our bodies, which are important to our overall health, and the fact that some people develop violent allergies to certain antibiotics.