Reducing information, not gathering it
Modern yardage books already contain enormous detail — distances to every hazard, slopes on every green, historical pin locations — so a caddie's real value is rarely finding new information. It is compressing everything relevant into a single adjusted number the player can commit to without hesitation, because hesitation over a shot is often more costly than a small error in the yardage itself.
Adjusting for what the book cannot know
Wind, temperature, how a ball is sitting in the grass, and even the adrenaline of a tense moment all shift how far a shot will actually fly compared with its book distance, and none of that is written down anywhere. A caddie's experience is largely a store of small corrections for these factors, built up over years of watching how specific conditions have altered outcomes before, which a player under pressure has neither the time nor the detachment to calculate themselves.
Managing the player, not just the number
Because a player's emotional state changes what club feels right in the moment, part of the caddie's role is simply keeping decisions calm and unhurried, sometimes by discussing the shot in detail and sometimes by deliberately not discussing it at all. This is why the same yardage and the same caddie can produce a different conversation on different days: the target is not only the flag but the state of mind standing over the ball.