content: In my experience .mp4 files have the best quality for the smallest file size. Why are other apparently inferior video file formats like .avi even used? I'm the first to admit I know nothing about video file encoding, other than some people are better at it than others.  But I'm always pleased when the file is an .mp4 because, as noted in the subject, of the quality and file size.  So I have to ask why isn't .mp4 used exclusively?