Well, let's say you wanted a border to be 300px width, 1px height, black, solid. then in the same horizontal axis, on the same border, 100px width, 1px height, white, solid - then again: 100px width, 1px height, black, solid.
Now you have a border that "changes" - an image would be easiest instead of writing a huge piece of code. I can see how border-image would come in handy.