It's not all just poopoo peepee stuff. A lot of plumbers do mechanical work in commercial buildings, boiler retrofits, repiping, construction. I'm in commercial HVAC and work with a bunch of plumbers whose skills and scope of work have great overlap with mine. I've also helped them out doing boiler retros/repipes. We offer similar services, subcontracting each other to our own customers, just they do more of the gas fired work and fix leaks in heating loops and I do the AC/Refrigeration stuff