If you have a screen with a very high resolution, in the current Mac OS X (and most other operating systems as well), everything will look very small because their sizes are measured in pixels and high-resolution screens pack more pixels into a small area. With resolution independence, you can choose how large things will appear regardless of pixel density — more pixels will just mean the picture looks sharper. This is especially useful for text, which can be really hard to read on high-resolution screens.