It's interesting to note because when I left portfolio school years ago nobody talked about Cannes. It was the One Show and CA. D&AD was something cool, but not the end-all-be-all. But over the past few years Cannes has really emerged as the show of shows. And I think I know why. It's not traveling to France (although that's a great boondoggle), and it's not because it's a world-wide competition (technically, so are most of the others).
It's because of the clients. Clients respect Cannes more than the other shows. Win a pencil and they're mildly impressed. Win a Lion, and you're retained as the agency of record and given a little more creative license on the next assignments. It lets them brag. It lets them feel (and sometime actually be) famous. Maybe it's the trips to France. Maybe it's association (or confusion) with the film festival. Whatever it is, clients love Cannes.
More and more, it seems if there's a show to aspire to, it's the one in the South of France.