Perhaps because you want to watch the video on the phone itself? The N8's HDMI output is really nice, but honestly, I rarely find myself wanting to play a video from my phone onto a TV. Most of the time, I just want to play the thing directly on my phone so that I can watch it wherever I happen to be.

VLC is a software based video decoder. Uses no dedicated hardware. For the Mac, your best bets for decoding video are:

(a) MplayerX: most up-to-date video player at the moment, supports Hi10P video and uses the Mac's hardware decoders.

(b) Mplayer2: still just a beta release right now, but does support chaptered MKV video files and also uses hardware decoding.

(c) Quicktime: doesn't support many video codecs or containers (although plugins exist to remedy this somewhat), but still does the best job of displaying what video formats it supports. And yes, uses hardware decoding.

(d) VLC: The last resort. A ways behind the times compared to other players (although a major update is supposedly in the works), the software-based decoding system means that it places a much larger load on your CPU, but it also means that the thing is infinitely tweakable. Video that is corrupted or just won't run any other way can often be made to work in VLC if you play with the settings just right...