Audio and video
Media jobs run through FFmpeg on the worker, and only when that engine is enabled. They re-encode or remux the file you upload. They do not improve a blurry source, and they do not keep edits that exist only inside a camera app.
Sound from a video, or a video you can post
MP4 to MP3 keeps the soundtrack and drops the picture. Use it for a recording where the image does not matter. MOV, WebM, and MP4 conversions are for a player or a site that wants a particular container. Balanced is the default quality. Smallest makes a rougher picture and a smaller file. High spends more bits on the picture. The same preset is what the job stores if you run it again.
GIF and MP4
A short silent loop is the usual reason to make a GIF from MP4. GIF is a poor way to store a long video: the file grows quickly and the color is limited. The other direction, GIF to MP4, is for a player that will not show the GIF and for a file that should be smaller than the animation. Trim the clip in the source program first if only a few seconds belong in the result.