In golf, caddies have three main duties: carrying clubs, making suggestions for each shot, and helping with club selection.
One of the caddy's main duties is to carry the golfer's bag, which contains a maximum of 14 clubs in addition to other essentials like tees, gloves, towels and ball markers. This is a very important job, as most golf tournaments require players to walk all 18 holes and carrying a relatively hefty bag for several hours would not be advisable for any player simply trying to focus on their next shot.
Caddies must also have extensive knowledge of the course, including the layout of each hole (slopes and sand/water hazards) and distance from the golfer's ball to the flagstick that protects the hole. This allows them to provide helpful advice with regards to distance, backspin and approach when the golfer is unsure of how to hit an accurate shot. Oftentimes, you'll see a golfer kneeling behind the ball alongside the caddy, which is simply their attempt to examine the natural contours of the ground and estimate how the ball will travel as a result.
Proper club selection is one of the most important aspects of golf and thus, an important job associated with being caddy. Since each club is designed to hit the ball a different distance, choosing the right one can represent the difference between hitting a good shot or landing the ball far from the hole. For example, a 9 iron is perfect for hitting the ball approximately 120 yards, while a longer club is designed for shots traveling about 160 yards. Although the golfer is generally aware of these things, having a caddy nearby to confirm club selections is extremely helpful.