This dropped shoulder line causes the armscye to form a distinctly diagonal line. The close fit under the arms allows more arm movement, even with the low armscye. It also prevents the dress from pulling up when you lift your arm or the sleeve from tearing out. This diagonal armscyce line also serves to further widen the look of the shoulder, which in turn makes your waist look smaller. The armscye should include very small self-fabric piping, to serve as reinforcement for this highly stressed seam.
Back seams are also very different from modern clothing construction methods.
The main difference is the use of side back seams to form a vaguely Y shaped center panel instead of a center back seam as is common today. The top arms of the Y lead out from the back of the armcyes, making sweeping curves to reach within an inch or so of each other at the center back. From this point they run parallel to each other until they end at the waist seam. This Y shape also serves to enhance the appearance of a small waist. Keep in mind, no matter how large the dress, ideally these parallel seams should end up no more than 1 or so apart.