Im just guessing this is to corner of a good amount of disk space so the app does not stop working from lack of space. An analogy is when downloading a torrent

Im just guessing this is to corner of a good amount of disk space so the app does not stop working from lack of space. An analogy is when downloading a torrent; the torrent blocks a chunk of space on your disk equivalent to the size of the file being downloaded. As the file is being downloaded it replaces junk in the blocked chunk. Again all this is my speculation.