There are video files that can come with more than one audio track. For example, an .AVI film with audio track1 in french and audio track2 in spanish.
On audio extracting (to a WAV file) or rendering, Kdenlive only assumes the first audio track and, as I know, there aren't any way to select which track to use. The ability to select audio track (and maybe various video tracks too?) could help in some works.