Because a C-clip, Christ clip, Jesus clip, etc... Is better for using on a shaft where it can be put on in a particular spot where space is limited and you may end up stretching it too wide and springing it if it were a normal snap ring type. Also.. where you may not be able to start on an end of a shaft to slid it up toward the snap ring groove.
One thing that comes to mind is for the governor linkage on a CAT 3114 engine.
They also use them as the lock stops for vertical blinds on the gear shaft across the top of vertical blinds.. Im not sure what the proper name for it is.. Sorry.
Couldnt tell you origins though. Looks like it may be UK...
Found this..
http://en.wikipedia.org/wiki/Circlip