The caddy’s role in most cases isn’t just to carry the bag, but also act as confidant, cheerleader, whipping boy, the angel on one shoulder and the devil on the other. Caddies help read the greens for putts, get to the course a day or two earlier to walk off yardages (every time you see the caddy or the player flip open a notebook, they are trying to ascertain the distance down to the foot), provide crowd control and suggest the club and type of shot the player should or should not attempt. Caddies were very important in the old days because most of them were local and had the knowledge of the greens and the course, it was rare for a pro to have a travelling caddy. When a pro picked a caddy from the local crop, it was usually X amount per day and/or expenses, with a cut of the purse depending on what place the pro finished. Standard was 7-10 percent for the win, 4 or 5 percent for second, etc.
These days most pros have a travelling caddy, one that they’ve established a trust with that really goes beyond just reading the greens and carrying the bag. Some of the bigger players treat their caddies just like an employee, with a yearly salary and everything. Of course the percentages they share in a win are smaller, but a steady paycheck and job security offset the occasional big payout.