If you add an audio or video clip into a presentation, you may only want to playback a particular segment of it and not the entire thing during the slideshow, so it would be useful to have the ability to do set these. Otherwise a user would have to use a video editor to crop out the segment they wanted played. Looking at the ODF format, we could add a <draw:param draw:name="PlayStart"> and <draw:param draw:name="PlayEnd"> to the media's <draw:plugin> tag.
Please add keyword 'needsUXEval' and CC 'libreoffice-ux-advise@lists.freedesktop.org' if input from UX is needed.