DVI doesn't improve graphics quality, more doesnt let it degrade.
VGA being an analogue signal is subject to degradation, so the quality of the cable can have an impact on the final image quality. DVI (and HDMI) being a digital signal doesn't degrade, it will work 100% or not work at all.
As well as DVI/HDMI just being able to host higher resolutions and frame rates (120hz) than VGA. HDMI can also carry an audio signal as well.