games look bad when you don't use the native resolution.
If you use the DVI output, (depending on your card drivers) you can adjust how the graphics information is sent out, can scale it with aspect ratio, display it 1:1 with the actual pixel resolution of the game, scale it full, etc.
Defaults to stretching it to fit on non 1280x1024 resolutions, which distorts the vision a bit because of the different aspect ratios.
(which is usually from 4:3 to 5:4)