It takes a lot of energy to turn liquid water into water vapor, and you can get a lot of energy back out when the water vapor recondenses. So we only need a thin (well insulated!) pipe to carry energy all over the building. In this picture, the steam comes in through the pipe at the right, and the pipe going into the floor carries away the condensed water. The energy that is given up by the steam as it returns to the liquid state is left behind in the radiator, available to heat the room.

Radiators were famous for loud clunking noises -- when they were turned on (by opening the valve at the top right), steam started to flow, which warmed up all the pipes between the radiator and the furnace (probably in the basement). Thermal expansion then pushed the pipes past obstacles, making noises.