Model making is really only useful for manufacturers of mass-produced identical pieces. It is usually only done when there is a requirement for making multiple molds over a very long period of time, as molds eventually wear out after a few hundred waxes are pulled from them. The master could be used over and over again to make new molds so that the manufacturer could continue to make a particular design for decades with no discernible differences in the finished product. As John says, it's really only useful if the design is to be reproduced many hundreds or thousands of times.
My Dad was a free-lance model maker in the 60's and 70's for a couple of manufacturers in Rhode Island. So while I don't have any extensive personal experience with the process, I did see it in use (and heard the swearing that could often accompany it)
One of the trickier parts of model making is the allowances for all of the different shrinkage rates during the mold making and casting processes. There was only vulcanized rubber for molds, RTV didn't exist so there was significant shrinkage in that part of the process. Then the wax injecting and casting processes also had some shrinkage and then further allowances were needed for finishing. Thicker parts would shrink more than thinner parts and thinner parts needed to be thick enough so that they would not be polished away during a several step semi-automated finishing system. There was also a requirement that the overall cross sections needed to be relatively constant throughout the entire piece so that casting issues like porosity were minimized. Another consideration was the ease with which the mold could be cut and then the ease with which waxes could be shot and pulled. A design with a very tight and complex multiple gallery setting structure would be frowned upon as it would be very difficult to make consistent molds from and could end up being problematic with non-fills and that kind of thing.
Remember, nothing was computerized, almost all automation was mechanical as opposed to electronic, so there was real art involved with model making. A model maker had to be able to look at a design, consider the requirements of the finished piece and then by eye and experience be able to determine where it needed to be thicker or thinner, how much oversize and where to exaggerate the shape of the setting so it would fit a particular stone size and what finger size to make the model so the finished piece would end up at a particular size. It was much more complex than it would seem to be at first glance. Quite different from making a mold of a piece in case you might get a call for another one some time in the future.