As was related by a golf club designer who worked for MacGregor Golf in Dayton, Ohio during the 1920's and 1930's:
All sets of golf clubs were custom built to the same club lengths prior to the introduction of steel shafts (invented in 1910 and legalized in 1926). Prior to that time hickory shafts were the norm and golf clubs were custom fitted/built to one single club length within a set based upon the static measurements of the individual golfer (wrist-to-floor measurement). The production and tuning of hickory shafts as well as the rest of the club making process was very time consuming and demanded the skills of highly experienced club makers. Obviously, this was an expensive process and could only be afforded by the wealthy which is why golf originally got the reputation as being strictly a sport for the rich.

The advent of the steel shaft changed all of this since this type of shaft could be produced by the thousands very cheaply in factories using unskilled workers. Large sporting goods manufacturers, lured by the huge untapped market for inexpensive sets of golf clubs, jumped in to bring golf to the masses. Their only stumbling block was that they could not provide single-length custom fitting in their mass production model. The solution came when it was suggested that these mass produced sets incorporate a 1/2 inch incremental step progression between successive irons and woods so that at least a few of the clubs within a set would come close to fitting any customer. In other words the incremental lengths in a set of conventional golf clubs today are the result of a miss-guided manufacturing decision made in the 1930s and not based upon any golfing performance criteria whatsoever.