Well, it's never a good idea to "divining rod" the intent of the NFPA. Because jackasses mainly are looking for one reason that they can rationalize around, so they can fabricate a reason why the Code doesn't apply to them. Really, they just don't want to follow Code, but they are not honest enough to call themselves outlaws.
So, it is a stupid game. In every rule I have researched, there are usually a variety of "whys" which overlap to provide safety.
But I'll give you one.
It stops you from building "unpullable" conduit!
Building the conduit first, as Code requires, keeps you honest. If you build it unpullable, you won't be able to pull it! :)
We had one person on here, we finally convinced them that a route between boxes would be best done as THHN and conduit. But they had to follow a zigzag on the building wall, and when they showed us a picture, they did not use a sweep in the inside corner -- they just put the exact same common "pulling elbow" they had used on the outside corner! The cover plate was facing the wall. (they make a special corner pulling elbow with a side door, by the way).
It was completely obvious that a) the person had assembled the conduit around the THHN wires, and b) they were completely oblivious that you would do it any other way. What a pain in the butt to do that, only to be told it's wrong.
It's much, much harder to do it that way
It really gives you the worst of both worlds. You have the "coils of cable splayed all over the place all day" like you do with a direct cable installation. And then you also have the "fidgeting with conduit" part too, except the conduit is much, much harder to do, because every piece must be slid down all the wires.
I do a ton of conduit work, and SMH... that's nuts. What a royal mess. Making it 5 times harder than it has to be.
Building empty conduit makes the job easier.
- The pipe goes up easily, obviously.
- Your work area is a space about 10 feet long - no bigger than the next stick. That means for instance you don't need to clear belongings out of the entire route, just the immediate <10' work area.
- Take a break anytime you want. Tidy up, and come back to it after lunch or next month. I have a conduit run somebody installed ten years ago but didn't finish the last 20 feet, it's not in anyone's way.
- It's all an erector set, which is particularly important to novices, who aren't going to get it right first time. You can disassemble and reassemble any way you like, with no loss of material except maybe some tubing. You can seek feedback off forums, and if something needs changing (like that pulling elbow pushed against the wall), you just change that little bit. It all un-bolts. (or if it's PVC just don't glue it yet).
There's no throne. Threading pipe over wires doesn't buy you anything.
Honestly, after you build the conduit, actually pulling the wires is a "victory lap". It's the easiest part of the job. I can't grasp why someone would want to make the job so much harder by trying to assemble conduit around wire. It's like doing direct cable, but with 10 times more mess.
Do people really think conduit work must surely be harder merely because it is different than what they're already familiar with?