what is the use of steam pipes underground big cities ????

Answer 1

In big cities, many buildings sometimes share the same steam boilers. This makes delivery of boiler fuel simpler, since there is a central location. This was especially important when the majority of boilers were coal-fired, which is when most of those buildings were built.

Answer 2

The steam is superheated and at high pressure (hence the explosion of a very old pipeline).
It is piped from the steam generating plant to a number of high-rise buildings in the area as a heating medium.
As the steam cools and condenses back to water, its pressure drops to near atmospheric and returns to the plant for re-vaporisation.

Answer 3

Many towns in Scandinavia in particular, have a system whereby heating is provided via steam. I am still gobsmacked as to why a big city like New York would still be doing this though... All that Asbestos as well... Dear oh dear, that will cost big time - health and money

Answer 4

To provide centralised heating to various buildings?