Their discovery and development is what has directly enabled our sudden rise from 2B to 8B humans in only about a century. Without antibiotics, we would likely still be under 3B humans world-wide. Yes, disease really did kill off a lot of humans back in the day.
Graph out the human population over the last two centuries, and you can even see the very decade when Penicillin use became widespread, along with doctors washing their hands and other basic hygiene tasks.