The Shoulder to Shoulder width is crucial for making wide necklines that don’t fall off the shoulder. It is also crucial for spacing the straps on corsets and bodices so that they stay on the shoulder and you don’t have to fuss with them all day.
The Shoulder to Shoulder measurement is taken from shoulder point to shoulder point. The point of the shoulder is where the bones are, basically. Optimally, it’s just a bit inside where your arm drops off (though that changes with time and poundage). Realistically, it’s the outermost solid part of the shoulder capable of bearing a strap or supporting a neckline. (I’ve seen women assume that their “problem” upper arms meant they had wide shoulders. This isn’t technically the case, and you’ll end up with your tops falling off your shoulder if you take the measurement across the torso at the widest part of your arms.) If your shoulder is more rounded and you’re not sure what area might be referred to as a point, you should be able to locate it by pressing your fingers into the flesh atop your arm and feeling for the mass where the shoulder bones come together.
The Shoulder to Shoulder measurement should also be taken to the back of the body. Those with pronounced scoliosis, osteoporosis, or similar conditions will find that the shoulder to shoulder measurement across their back is significantly wider than the measurement across the front. (As a hint, you’ll want to fit your larger measurement if you’re making a jacket or something that completely encloses the back of the figure, or your garment will be to tight across the shoulders in back. If you’re making something with a low neckline, however, fit to your smaller front measurement or you may have troubles with things sliding off your shoulders.)