Through reading this page you will be instantly familiar with the character of the work performed by the hydraulic plant looking for new employees.
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
They can be associated with connecting new sanitary facilities or carrying out small repairs.All such work can be carried out also by plumbers self-employed.
However, this should not affect their quality, and sometimes it turns out that the plumber running his own business applies doubly to perform the outsourced tasks themselves.