Much of the plumbing work in populated areas is regulated by government or quasi-government agencies due to the direct impact on the public's health, safety, and welfare.
WPC has subsequently appointed a representative to the World Health Organization to take forward various projects related to Health Aspects of Plumbing.Źródło: https://en.wikipedia.org/wiki/Plumbing
Plumbing equipment includes devices often hidden behind walls or in utility spaces which are not seen by the general public. It includes water meters, pumps, expansion tanks, backflow preventers, water filters, UV sterilization lights, water softeners, water heaters, heat exchangers, gauges, and control systems. Specialized plumbing tools include pipe wrenches, flaring pliers, pipe vise, pipe bending machine, pipe cutter, dies and joining tools such as soldering torches and crimp tools.New tools have been developed to help plumbers fix problems more efficiently.For example, plumbers use video cameras for inspections of hidden leaks or problems, they use hydro jets, and high pressure hydraulic pumps connected to steel cables for trench-less sewer line replacement.Źródło: https://en.wikipedia.org/wiki/Plumbing#Equipment_and_tools
The houses usually they exchange seals and pipes, which has already gathered a lot of dirt and hair and suds.Similarly, in flats, where sometimes you also need to fix the toilet flush.
In addition, home handymen often deal with repairs washing machines standing in a bathroom or dishwashers standing in the kitchen.
So they can save some money and demonstrate their efficiency in front of family.