Powering the city, that’s what (or at least powering Manhattan up to 96th Street, which is why you never see the steam vents out in Brooklyn). Or as ConEd puts it:
Steam power from Con Edison is as much a part of Manhattan as subways and Times Square. The first steam generation plant began operating in 1882 – six months before the first electric service. Today, steam power has grown to play a major role in the life of the city. More than 100 miles of mains and service pipes make up the Con Edison steam system. The pipes deliver this clean, efficient energy source to about 2,000 customers from the Battery to 96th Street. In fact, the Con Edison system has become the largest steam district in the United States – larger than the next four U.S. systems combined as well as the largest steam district system in the world.