GCC Visibility

I understand the distinction between public and private variables and functions. I don't have a grasp about why this gcc visibility thing makes a difference. Why is enabling this good for me as a builder/packager? As an end-user? I get the feeling with hiding the private non shared stuff that executables and libraries are smaller in size, which theoretically reduces load times, which theoretically should improve response on the desktop. But how much? A layman's explanation is much appreciated. :) Darrell