Here are some use cases I thought of:

go to the top level domain directly instead of hunting and hoping that they have a link there

going up several directories on an ftp site

go from reddit.com/r/starcraft to reddit.com/r/nba

go from a foo.github.io/bar docs page to github.com/foo/bar instead of hunting, and hoping, for a link

increment the page number on a blog by several instead of clicking next repeatedly

prepend http://www.google.com/url?q= to NYTimes urls to avoid paywall

add ?limit=100 to see more comments on reddit comment page

going to a new topic on wikipedia