My guess is so that HD videos / movies downloaded from Apple will look good on your new iMac!
I honestly think that Apple will silently update the iMac, MacBook Pro and Mac Pro with a BTO Blu-ray and/or HD DVD drive option after Leopard ships. The costs are coming down on the drives and Leopard is the perfect time to add the playback support at the software level. It makes it even easier if they have hardware already out that has the capability (at the graphics level at least).
There are a lot of people who use the xBox 360 HD DVD add-on with their PC's. If Leopard adds the software support you could probably just use one of those...
there currently are not any HD movies on iTunes, but my guess for why it is HD, is to help with editing HD video and just to make the screen sharper.
Why does Apple include an HD compatible video card if they chose not to include a Blu-Ray or HD-DVD drive in this round's updates with the iMac?
OK, so then here's a question. Assuming Apple would release an internal BTO option for the iMac, would us "already iMac owners" be able to take our computers to an Apple store and have the internal drives swapped for the new HD drive?
The whole deal with HD is really just a TV thing. I don't want to get to technical, but, Non-HD TV's have lower screen resolution that dates back to the 1940's. New technology allows TV stations to broadcast an improved signal that is much higher resolution. This higher resolution is called HDTV. Computer screen resolution has been much higher than TV for some time now (many years).
My earlier post was meant to be "tongue in cheek". Certainly, if you download (buy) a movie from the Apple iTunes store, it will be much higher resolution than what a non-HD TV can display, which is why Apple suggests your TV be connected to a HDTV.
BUT, there is one very important feature of this graphics card (that, in my opinion, grants it the right to be called HD): it supports HDCP. (Wikipedia article In a nutshell, because this graphics card supports HDCP, you can play HD-DVD and Blu-ray movies at full resolution.WTF? It is not an "HD" card its just a marketing gimmick for something videocards have been able to do for years.
Expel the High-Definition term out of your brain and replace it with the term resolution. Remember good ol' resolution? Yeah so much for the HD craze.
Not true. The resolution of the iTunes content is not higher than SDTV. It is actually just a little lower, that is why Apple says "near-DVD" quality. iTunes store material has a maximum resolution of 640X480 - SDTV is around 720X480.
BUT, there is one very important feature of this graphics card (that, in my opinion, grants it the right to be called HD): it supports HDCP. (Wikipedia article In a nutshell, because this graphics card supports HDCP, you can play HD-DVD and Blu-ray movies at full resolution.
P.S. I hope there will be an offering for us early adopters to upgrade our internal drives to Blu-ray. If not, I'll just have to get an external case that uses FW800 (OWC makes some great external cases, BTW). I really want to watch full HD movies on my full HD iMac. Mmm... 1920x1200... (1080p is 1920x1080)