The revision graph is a nice way to get a quick overview of your project. However you can not share the graph with other people unless you save it as an image and pass the image on.
If the revision graph is small, then saving the graph as a png image will do just fine. But as with all images, a png does not scale very well. And if the revision graph is already big the resulting image file can get quite big as well. It can even get too big to show.
While you can save the graph as a wmf file which is much smaller and can scale perfectly, the wmf format is Windows only and won't show in e.g. a browser.
Since TortoiseSVN 1.7 the revision graph can also be saved as an svg file. An svg file is a vectored format which scales perfectly, is relatively small and can be shown in all modern browsers.
To illustrate the problem with big graphs, I saved the revision graph of the TortoiseSVN project as a png file and also as an svg file.
The png file is 950x8116 pixels and 250kBytes in size. While the size on disk is not too big, the 8116 pixels in height are quite big.
The svg file however only requires 161kBytes on disk and can be resized without getting pixelized.
You can compare the two different formats right here in your browser:
Try resizing the images in your browser (usually using Ctrl-mousewheel or with Ctrl-+). The png image gets pixellated real fast, while the svg file scales fine.