Steam is used on city, industrial and campus-wide central heating systems where going longer distances without pressure and temperature drops that a hot water system would have. Steam also requires on-site boiler operators who perform maintenance operations, state pressure vessel registration, etc.
Newer hot water or "hydronic" systems use a circulator pump, but some gravity hot water systems were designed for use in pre-WWII homes.
Cast iron radiators can be used with either HW or steam but with steam, a low connection is used only. Once steam condenses in a radiator, it returns to the boiler as water, again out the low connection. HW often goes in the low for inlet and high side for outlet.
https://www.thisoldhouse.com/2109707...or-performance
Below, an ornate radiator in a 140-year-old residence outside in Norristown PA
3rd floor radiator.jpg