tracker-writeback is not supposed to be run by the user since it is started by its .desktop file when the user logs in. It can also be started manually of course for debugging purposes. You can not run more than one instance of this at the same time.
tracker-writeback writes metadata from the Tracker database back into files only. Currently support is limited to XMP metadata (which covers PNG, JPEG, TIFF, MP4 and 3GPP formats), play lists (which covers MPEGURL, SCPLS and IRIVER formats) and taglib supported mime types (which covers MP3, MP4, OGG, WAV, FLAC and some Windows media formats).