Rereading the code myself, I see that what the function does is prepend "http://" to a word if it looks sufficiently like a URL but doesn't have the <scheme> part.

Try the very useful command "git whatchanged -p src/fe-gtk/fe-gtk.c" to give 2662 lines of output, of which lines 970-997 appear to show the introduction of the WORD_PATH macro, by bviktor on Sat Oct 13 2012.

With my humongous URL-and-etc parsing changes it may well be that we can get rid of WORD_PATH, but I haven't studied the problem much.