The regulator, often called a Doctor's Watch, was used by them to measure a patient's pulse. Anyone that needed such fine timing to the second in the 30's and the 40's would buy one. This included technicians and engineers. It is a very complicated movement, and the service costs are high, to that I can attest. Many brands made them, and most are not cheap today. I think the Chronograph, once affordable and widely available, made this this complication obsolete. In today's finer watch brands (and cheap Chinese models too) you can find this recreated, mechanically and in quartz.
A less complicated, and more widely found version of the Doctor's Watch, was the two dial version. A tank (sort of) that had a upper circle for the hour/minute hands, and a lower circle for the seconds. But this watch was much more difficult to time a pulse as the lower dial was so small.
Well, this is my understanding anyway. <shrug>
Me? I bought them both because the Zodiac versions are rarely seen today. Once I get #2 back I will post a picture. Funny thing is that all three are radically different in chapters, dial, and hand design.