A talented caddie is extremely helpful to professional players. He advises on club selection based on terrain, wind, humidity etc. He also knows how to read minute variation on greens. Even more important, he keeps the player relaxed and focused.
Adam Scott started winning again after changing to a long putter. My own scores have dropped since switching to a long putter.
Nick Faldo won with his girlfriend caddying for him. I think a good golf coach can really help a player improve, but most caddies are simply guys who wanted to play but cant, so they carry the water for the real players. Also, not a bad gig for those who hook up with a big money maker.