We humans began developing sewer systems long ago. We have been working on that project for perhaps six-thousand years. As our populations increased and our cities grew, it became obvious that something had to be done. Eventually, we developed plumbing as we know it, and plumbers were required to work on it.
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Now of course, we have sanitary sewers by agreement and decree as well. In large cities we have large sewer systems, that have grown vast enough to even accommodate the urban legend of giant alligators in the sewers under New York City, ever ready to dine on the unwary city worker or other unfortunate. In your town you have an excellent sanitary sewer system, which hopefully leads to an advanced treatment facility, and not to the river. From a short walk in the woods to a low-flush toilet; from the gutter to a computer-monitored sanitary sewer system; we have come a long ways. To connect you to that sewer in your hour of need you have plumbing. And to keep that plumbing flushing and flowing we have plumbers.