If you use a screen size, for instance or 1440x900 then you'll be tempted to design the background for the theme at 1440x900 which will look great. But if somebody wants to use your theme but uses 1920x1200 for their display, then they will need to resize (upscale) your original background which will result in a lesser quality image.
So if possible, design your theme using the highest resolution you can. This then allows for good image quality when resizing down. If you haven't seen it, you might find some useful info in the
Creating Themes guide.
We resize the background image to fit our screen resolutions because Chameleon will not re-scale images for us. So if I use 1680x1050 and use a background of 1440x900 positioned at 0,0 then I will have a blank area of 240 pixels on the right, and 150 pixels below.
It's up to you what you do, but if we choose to design a theme that uses a background then we have to do this to make it user friendly to others.