Off the top of my head, not many. One might be densely cross-linked documentation markup stored on GitHub but officially published elsewhere. If you have reason to use absolute paths and want your docs navigable everywhere, you need to do something to about the `/$USER/repo/blob/master` vs. `/doc/wiki` (or whatever).

Or, to ease writing said docs, you might wish to resolve "fake" urls like `wiki:Batman`, which should point at https://en.wikipedia.org/wiki/Batman ...which I guess still amounts to prepending, doesn't it?