I don't think you quite understand. At the course, you are practicing for just that course. At the field, I can try different shots with my practice basket. I work on overstable, understable, approach shots, mids, things like that. Plus, I have my main bag, my back up bag, and my cooler sitting right there, so I have 40 discs with me, and I can see which ones are good for my bag, and I can throw over and over to see how two discs might be the same or different. If two discs are the same, I know that one is back up and one is the main.
If you could haul 40+ discs around the course and work on as many shots as I can, I would say they are the same, but you are not the same. You are working on shots for that course. I am working on shots for every course. So even though you believe that practice on the course is good, and it is , you still are limited on how much you can do. With my practice basket, my measuring tape, my cooler and over 40 discs, and a field that is over 500ft long, and over 225ft wide, and have more I can do, and more I can practice with. And, I never have to stop to let someone play through.