I used to train with somebody who didn't even let you take open classes until you had at least 1 novice leg. >.< That mentality stuck until I was taking classes with my Jacks and the people teaching the class sort of made it clear how closely related all the levels are and even now it's particularly clear now that AKC dragged facets of utility into open.
With novice you've got your off leash heeling and your recalls. If you do not have that downpat, then you are not going to be able to jump into open where all heeling is off leash (you can't even touch your dog's collar in the ring unlike in novice where it's allowed to lead the dog from exercise to exercise by the collar, don't rely on that or do it, but you can if you wanted to) and you go from the one recall in novice to three of them.
If your dog is iffy on off leash and gives you crooked fronts each time, then you will have your points drained away in open.
With open you have your off leash heeling, recalls, retrieves, "open signals", and jumps. If you struggle with all these areas, you will not be able to make the jump into utility which bumps up the difficulty with heeling, signals, and recall exercise without any verbal commands, two retrieves (to marked objects, not thrown), you have a stand that you hopefully trained and maintained from novice level but with more hands on exam + call to heel, and then the jumps are their own level of difficult with the send away and jumps from an angle, and I can go on and on...
But main thing is that novice is the foundation for the rest of obedience. Everything you do in novice will reappear at the higher levels but at a greater difficulty in a "But wait, THERE'S MOREEEEE" kind of way.
BN has become pretty standard as the intro to novice, because it has the on leash heel (including f8), sit for exam, and recall as you would have in regular novice. Well, you have a stand for exam in novice, but if your dog can't handle a sit for exam then standing will be problematic. If you do not have these exercises downpat to Q in BN, you will not be able to make the jump into novice.
Rally... I do not think was ever intended to be an intro to regular obedience. It was more about giving people with retired dogs who could not jump full height something else to do rather than completely retire + they also wanted it to be a cross between obedience and agility. When it was initially introduced at the place where I trained, I'm not sure if the exercises were different at the start or what, but we had more exercises and jumps from agility set up on a rally course. You had more overt handling and talking allowed, etc.... which made it more difficult to then jump into novice because you were shaving away everything for the most part that made rally easier than regular obedience.
Now they have bumped up the difficulty in rally, so I'm not sure how it compares or leads into obedience.... other than people do have issues fading the handling and verbal praise/encouragement/etc from rally to regular obedience. There's people who might get a 100 in rally struggling to Q in the 170's.