Good caddy removes a lot of decisions from the player when under pressure. 2nd opinions. Physcologists. Some will read the putts. They would also do a lot of groundwork and prepare own yardage books for players based on the route around the course their player is likely to take
I couldnt say if value is overstated or not. Based on the long term relationships lot of the v top players have with their caddys then I assume it is worth a significant amount.
Steve Williams is an utter tosser but he has been hired and been hugely successful with a host of top golfers over the years. They are all hiring him for a reason