The reason that's interesting, at least to me, is that this is a prime example of what's known as combined heat and power generation. It's an old idea, but one that's making a resurgence as bills for all our petroleum-dependent energy sources--heating oil, natural gas and electricity--continue to climb. As we all know, the easiest way to "generate" more energy per dollar spent is simply to conserve.
You might also wonder, as I did, why the heck these pipes are pressurized even in the middle of July--clearly the steam isn't being piped into radiators. Here it turns out that an additional cleverness has been introduced into the system: buildings in the financial district use the steam to power the compressors that run their massive air conditioning units.
The whole thing is rather brilliant--a model of re-use and smart urban planning--that is until disaster strikes. Apparently there have been lethal steam pipe explosions before, the most recent in 1989 in Gramercy Park.