One of the things biologists are interested in is change over time, and we compare genomes to see where changes have occurred between two or more species; one of the reasons we're interested in the chimpanzee genome, for instance, is that it is close to ours, and what scientists are doing is comparing the two, looking for the key differences.
There are other things that change over time that lend themselves to these sorts of analyses. Darwin's On the Origin of Species went through six editions during his lifetime, and it wasn't a static document at all — he revised, sometimes extensively, and he added new material, sometimes in response to new data, sometimes in reaction to public and private concerns.What Ben Fry has done is taken the text of all six editions, compared them, and color-coded the words by when they were added. Then he rendered them in teeny-tiny print and splashed them up on the screen so you can see when and where changes occurred in Darwin's text. It takes a while to load, since it is loading the full text of six editions of the Origin, plus annotations, but then you can just move your cursor around over the blocks to read and see what he was thinking. For instance, one thing that jumps out at you is the huge block of red in the middle of the document (not in screengrab), a whole large section that was added in the sixth edition.