mwillems is correct. Let's say I'm at a hotel and want to watch a movie from home. Normally I'll start it playing, pause it, do something else for awhile, then come back and hit play and hopefully enough has been streamed at that point that I can watch the movie without issue. Sometimes bandwidth is lower than I expected and I hit buffering partway through. It would be nice to be able to see just how much of the current file is now "local" so that I can better judge when it's safe to start. Even better would be an estimation of completion time based on average streaming rate. Even better than that would be an additional indicator that, based on avg streaming rate and length of the video, it's safe to start playing the video (like a light that goes from green to red).
It's really only an issue with video as the bitrates for those exceed my upstream rate whereas is with audio it's no problem (as long as I'm downconverting flac, etc).