The simplest reason, "why a video card," is that my Gigabyte MA770T-UD3P
motherboard has no on-board graphics. I prefer nVidia graphics, and going for
on-board nVidia graphics would have limited my choice of motherboard.

I selected the motherboard by going to the Newegg site, and picking the
motherboard which could host that CPU, with the highest user satisfaction rating
at a reasonable price. I wound up buying the motherboard locally, but I bought
the graphics card from Newegg. It was not at all expensive, $51 Canadian
including taxes and shipping.

Another advantage of a video card is dedicated memory instead of sharing main
memory. That's more relevant if you have less main memory, such as 512 MB.
Plus the flexibility of buying as much performance as you want.

Windows 7 includes a program which rates various system components in order to
predict whether the performance of different games will be satisfactory. That
program gives my video card a lower rating than the other components, and gives
the RAM the highest rating.

To be completely honest, video conversion and editing is my only application
which demands this kind of system performance. However, a couple of other apps
use a surprising amount of CPU, based on watching System Monitor. One is Skype,
when used for videoconferencing, and the other is Miro, when you are watching
video at the same time as downloading.

My wife is currently on an extended trip to China, so videoconferencing is my
top "must-have" application.