A few years ago, redditor Josh Wright converted an old treadmill into a belt sander.
He stripped apart the treadmill (which was no longer useful for its original purpose) and then attached a new 1-HP motor. Wright built a frame suitable to holding up the entire unit as well as make it practical to use. He then experimented with different adhesives for the belt and found that ordinary shellac was the most effective. Wright used the shellac to add sandpaper sand to the belt.
It sands quite effectively. I can see it being useful for very large projects.
You can watch this complete build process here.
Most people use the treadmill's existing motor and speed controller, because they typically offer good belt speed and power output, a variable speed belt is highly desirable, and it greatly simplifies the build.