The purpose is, for movies/videos for which no actual HD resolution exists, to enhance the image into a higher quality and size. If done properly, it can actually look quite good.
Quite frankly, if you never thought of it this way, many/most DVD and Blu-ray players and televisions will do upscaling of an SD signal into 1080p HD, or on a 4k television like I have, it can also upscale (and usually these devices do it automatically with an SD signal) to a near-HD look.
Since your disc players, tv's, and sometimes also your media players on your computer all can do the same thing, I don't understand why people question an upscaled upload here. Some people actually have bitched about upscaling in some torrent comments.
Even some cable tv and streaming signals online are actually upscaled to 1080p HD if the studio owning the movie/show hasn't done its own HD master from original elements.