A HDMI cable allows you to use high-definition output in many devices, such as a Blu-Ray player, HD DVD player, not just the Xbox 360. It will only work on compliant devices, however, such as HD monitors or TVs.
The other answer was rubbish so here it is if you want to increase the quality of your game graphics get one and plug it in to the exterior of your console, personally i think it doesn't do anything...
The HDMI cable is made to simplify your entertainment system. It also gives you maximum high-definition. Just plug one end into your Xbox and the other into the back of your HDTV and your done...
not as high quality in your gaming experience, hdmi cable is a lot better and a lot more clear. The AV cable is used for a standard definition TV (Tube TV about 480p), while the HDMI is used for a HD...
No, you can just have the standard scart or component cables but HDMI will give a better picture with better sharpness and more clarity.You can view at 1080i and P with a HDMI cable.Hope this helped.