A lot of people use it to cover up crappy finishes or use it on pipes they've welded up. I heard it helps hold the heat in the pipes like ceramic coatings, but don't quote me on that.
In the racing world it was used to keep the heat in the pipes, but on our scoots I don't think it will make a noticable gain in power or speed. But it sure helps covering up rusty old pipes, as Travis wrote..