For longer documents, especially with multiple different blocks like code or images, it seems to be actually useful. It gives a fast impression on the length of

For longer documents, especially with multiple different blocks like code or images, it seems to be actually useful. It gives a fast impression on the length of the site, where you are and where the interesting and boring landmarks might be. But this also demand that the content is centered, and hos enough free space on the side, which is not that uncommon these days.