As men's clothing has become much more fitted over the past 15 years, the difference between too baggy and too tight has eroded the generous tolerances that clothiers had been accustomed to in the past. While enough men are fortunate to be able to buy off-the-rack, almost everyone can benefit from a custom fit.
For off-the-rack jackets, all measurements are extrapolated from the chest - everything from shoulder width to waist size. For shirts it is even harder as the neck size is used to build the body pattern. For example, if you have a thick neck and narrow shoulders, you are basically stuck wearing shirts that hang off of you.
Even if you are one of the rare individuals that has a nearly perfect fit off-the-rack, you most likely are looking at a limited number of choices in fabric, lapel style, number of buttons, etc.
In order to correctly fit a garment, a multitude of skin measurements are required to account for all of the nuances of the body. It is also necessary to account for posture, such as the curvature of the spine and the angle of both shoulders.
After the measurement process is the fun part: selecting fabric and details.