They may rely on both the exchange of a faulty seal and at the connection of the new tap.
A boiler is a closed vessel in which water or other fluid is heated. The fluid does not necessarily boil.
(In North America the term "furnace" is normally used if the purpose is not actually to boil the fluid.) The heated or vaporized fluid exits the boiler for use in various processes or heating applications,12 including water heating, central heating, boiler-based power generation, cooking, and sanitationŹródło: https://en.wikipedia.org/wiki/Boiler
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.