Very simple.
Owners could open the case and place it on a nightstand for easier viewing. The dust shield over the movement helped keep out the dirt. - Genious.
Don't know this for a fact, just seems to make sense. The hinged cases were most likely not designed so that they could stand up for viewing. Probably done that way for manufacturing reasons. It would be easier to accomodate different sized movements if all you had to do was machine out the center flat part of the case. With a regular case a lot more work would be required, new stamps and pressing procedures. Look at all those early Bulovas which came with 9 or 10 series movements. Just a guess on my part.
Patented June 10, 1924 Arthur Marrotti of New York N.Y. Quote from approx line 14
"Watches of this type are subjected to unusual exposure, and are therefore liable to injury from dust or other elements, and it has been my object to devise a construction which while compact and of artistic appearance will also comprise complete resistance to the entrance of dust to the works, and a transparent covering therefor which will permit a view of the movement but prevent tampering therewith by inexperienced persons."
That was Arthur Marrotti's objective. Bulova thought it was a pretty darn good idea.
Wayne