The QVGA compilation enables you to use one set of images - those that fit QVGA screens. The QVGA compilation multiply the pixels for VGA screens so you don't have to do any special programming for the VGA.
The QVGA compilation can be used by both types of devices.
When you use regular compilation for VGA screens - then you have to cater for the images and create larger images to be used with the VGA, and decide which to load with the if statement checking the screen size
(height or width)