"If you don't know the distances you hit your clubs—and most golfers don't—then you're giving away a lot of shots on the course." - Gary Wiren
We've all been standing on a tee box of a long par-3 and heard one player ask another, "What club are you using?" His friend will say, "I'm using a 7-iron." Then the one who asked the question will go to his bag, put away the club he has, and grab a 7-iron.
This is almost always a bad idea. Club choice in golf should be a very intentional and confident part of our game. Each player has his or her own unique swing speed and power, and we need to have enough consistency to know what club we should use for any given shot. To use a club that fits someone else's game is not only a mistake, but a show of insecurity in ourselves and our abilities. It is the golfing equivalent of peer pressure.
The reason practice ranges are equipped with flags of varying lengths, each marked clearly, is so players can work with their clubs to learn how far each club carries the ball. Any good practice session will consist of a focused attempt to zero in and strengthen those distances until we have an intimate feel for our ability with each club in our bag. Then we can take that knowledge to the course and hit more greens and lower our scores.