I will give my view as someone who paid for a self supported Adventure Cycling Tour and paid for two fully supported tours in Europe with a tour company.
The European tours had guides that repeatedly worked as guides on those specific tours. If you asked a guide a question, he or she almost always had a quick answer (how much farther today, where is the nearest ATM, any good places to buy some coffee, etc.). When they did not have the answer, they would try to get an answer for you pretty quick.
The Adventure Cycling tour I took, the guide had guided many different Adventure Cycling tours, but he worked on a different tour each time. I think he was basically using the guiding position to go on a subsidized vacation as he took different tours each year. He had a binder of important stuff to know - where the campgrounds were, where the grocery stores were, possibly other things like location of post offices, bike shops, where to buy camp stove fuel, or medical services. Thus he had all he needed to know for group logistics. But, if you asked him a question on where to find something else that was not necessary for his guiding duties, he was clueless. One day he was saying that there would be showers at the campground, I told him I was at that campground a week earlier (I stayed there for a few days before I joined up with the tour group) and I was unaware of any showers. He assured me that there were showers there. We got there and there were no showers. Knowledge of the presence or absence of showers for one day is not a big deal, but it is an excellent example of what it is like when the guide has never been on that route before. I enjoyed the trip and am glad I went, but that experience changed my expectation of what the role of the guide is on an Adventure Cycling tour.
As noted, budgeting, logistics, possibly dispute resolution are probably the important points. On a self supported tour, the guide went to the grocery store with the people tasked with cooking that day and he told us his job was to ascertain that we had enough food for the group. I assumed reservations for campgrounds/lodging were made by the office, thus I assumed that was not a duty of the guide. Guide might need to have some first aid expertise and certifications.
I am good at budgeting, good at logistics, taught first aid many years ago, but quite frankly when I go on a trip the last thing I want to do is have to worry about others. I would never consider being a guide.