The Before/After feature shown here is a great way to show and illustrate certain changes, such as over time. It can also be used to show the same thing in different ways. Rather than simply using two different before and after images of whatever or wherever has changed (or is different), the two images are used, but combined together as one, with a sliding divider that the user can slide it back and forth to reveal (and hide) more and less of each image.
The examples shown here will illustrate this best. As you can see, the result communicates the differences and/or changes much more effectively than the same two images side by side.
photo courtesy of National Geographic
The above image is where you can see two different views of the same thing (in this case a map view of Manhattan, and an aerial view. The images to the left are also of Manhattan Island, but in this case we have an illustration of what it would have looked like long ago, before being developed, and then after being developed into the New York City we know today.
Here is an example done by a blogger who created the 2nd background image to show why he thought the new Conan O'Brien set looked like the Mushroom Kingdom graphics from the Super Mario World game.
If you visit the original blog post on SeriousLunch , you will see that the blogger did not have this functionality. So he first uses the two images separately, and then combined into one animated GIF file for a now very dated effect (and thankfully no longer necessary because it is annoying).