A not-uncommon workflow of mine is to have more than one window looking at the same buffer in several different places.  Often in one of these windows I switch to some other buffer (perhaps a shell) and then want to switch directly back to the same place I was just looking at in main buffer.  However since my main buffer is already visible in another window, C-x b (switch-to-buffer) does not default to that buffer, but prefers a buffer that is not already visible.  This customization setting allows users to override that behavior so that switch-to-buffer does not prefer buffers that are not already visible.

Jefferson