A licensed plumber means the person completed the necessary education or training required to work as a professional tradesperson. Hiring a licensed plumbing service means the people working on your building are knowledgeable about water and gas pipes. If you use unlicensed workers, you run the risk of them damaging your home even further and endangering you by doing faulty work.
We called Mr. Rooter because we had a leak from our kitchen sink and they responded quickly to my surprise. Mike Farnsworth shows up and gets right to work and doesn't give us an estimate for the work before he started. He ended up replacing a broken pipe in our basement that took 15 minutes and he plunged our tub and sink as well. He wasn't even here for an hour and he charged us $650 dollars! We refused to pay that much because it wasn't reasonable, so he took us to court and he ended up lying in front of a magistrate claiming he worked for 2 hours and did all these repairs, even though after he had left the sink was still leaking and the drains did not work at all!