Caddying is a real art form.
They show up at a tournament a few days early and map out the entire course while taking into consideration "their golfers" game. They organise everything to do with the golfers equipment, including maintainance, new clubs and even down to making sure they have enough balls and the correct types of tee's.
On playing days, they advise the golfer of yardage, potential hazards and club/shot selection. They also do a bit of amateur pyschology and generally manage the players mood as best they can.

Its only the top 5-10% of caddies that lead a celebrity lifestyle. The rest struggle to make ends meet more often than not.

This book is worth a read, totally hilarious and tells a true caddie's story.

Four Iron in the Soul: Amazon.co.uk: Lawrence Donegan: Books