What is the main use of antibiotics produced by useful microbes?

Antibiotics like penicillins produced by useful microbes are used to kill disease-causing harmful microbes.Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow. He found out that it was due to a chemical produced by the mould and he named it Penicillin after the mould Penicillium notatum.