Remember, the shaft's weight is the number one factor that controls the total weight of the entire golf club. Lighter total weight equals the potential to increase the golfer's swing speed, which equals the potential to increase the distance of the shot.
The average steel shaft today weighs between 115g to 125g (4.0 to 4.4 oz.). Put that together with a typical 195-gram (6.9-oz.) driver head and a normal 50-gram (1.75-oz.) grip and you have a total weight for the driver of some 365 grams (12.9 oz.).
Most graphite shafts for drivers today are made to a weight of around 65-70 grams. Assemble that with the driver head and the grip and the total weight of a typical graphite shaft driver will be about 11 oz. That 1.9-oz.-lighter total weight (compared to the typical steel-shafted driver) can mean as much as 2-4 mph more swing speed for the golfer, which in turn translates to about 6-12 yards more distance.
Makes it sound like all golfers should be using graphite shafts in all their clubs, right? On the surface that is true. However, some golfers who are very strong physically, and/or who are quick to very quick with their swing tempo, need to have a little heavier total weight to help them gain a little more control over their swing.
In addition, steel and graphite shafts are totally different in the manner in which they transfer the vibrations from impact up to the hands, which in turn affects the feel of the shot. Simply stated, some golfers prefer the more crisp, sharper feel of hitting the ball with steel shafts, while some prefer the softer, more dampened feel of graphite.
If gaining more distance is a primary goal for the golfer, they should definitely be fit with the proper graphite shaft design in their woods and irons to match their swing. On the other hand, if distance is not the main focus for the golfer because they already have a high swing speed, if they like the feel of steel and their swing tempo matches a little better to the higher total weight steel shafts bring to the clubs, then steel is the better option.