I thought they came up with the herbst as a replacement because no one would actually wear headgear.
Yeah, getting kids to wear HG is hard (and I really don't blame the kids!)
So, I just did a bit of searching to find out a bit more about the history of the Herbst, found some interesting background at
https://dereferer.me/?https://glidewelldental.com/company/blog/how-the-herbst-appliance-became-a-snoring-device ....
"In his 2003 paper 'History, Background, and Development of the Herbst Appliance,' which cited an unpublished survey of the top six major orthodontic laboratories in the United States, Dr. Hans Pancherz reported the Herbst appliance had grown to be the most popular functional appliance for the treatment of Class II malocclusions."
"Dr. Emil Herbst (1872–1940), who invented his eponymous appliance to correct Class II malocclusion, made many contributions to the field of dentistry; his most famous design appears in the literature in 1909 for the Fifth International Dental Congress in Berlin. To remove patient compliance as an obstacle to treatment, the Herbst appliance was designed as a fixed device that acts as a hinge between the maxilla and the mandible. The hinge was made of silver or gold tubes with corresponding rods that forced the mandible forward, but allowed speech, chewing and swallowing."
A Class II malocclusion is, of course, the issue where a patient's lower jaw is too small, so the herbst is there to encourage lower jaw growth in teens.
(That article goes on to talk about using a herbst to help with snoring issues: in many cases, pulling the lower jaw forward opens the airway a bit, and stops the snoring, and there's several appliances, herbst-based included, to do that).
So, back to the headgear, I had it explained to me ages ago by a friend who was an ortho, that it was mainly used to help "anchor" the molars (if you use power-chains or elastics to pull the front teeth back, due to the simple laws of physics, they also pull the back teeth forwards! The "by how much" apparently relates to the number of roots.... molars typically have 3, sometimes 4 roots, premolars typically 2, and front teeth a single root), but also to push the molars slowly backwards.
I think some use of headgear has been replaced by lip-bumpers (still not easy to wear, I suspect, but probably easier than HG!).
In many parts of Europe, the preference has been for removable appliances (rather than Herbsts), and
https://dereferer.me/?https://www.bos.org.uk/Museum-and-Archive/Collection/Functional-Appliances talks about the activators, twin-blocks and Frankel appliances. I often wondered why we used removables, and that article explains: "Functional appliances gained popularity in Europe after World War II as the precious metals used for fixed appliances were difficult to obtain. Many designs were produced in the 1940s and 1950s including those by Bimler, Schwarz, Stockfish and Frankel."
So there you go, far more information than you were wanting!!!!