Why? You ask Why?!! WHY?!!!
I’ll tell you why!
Because... um... Well, you can... um...
Heck with it. I have absolutely *NO* idea why you would want to swap the barrel only and have to re-zero, instead of slapping on another zero’ed upper and being instantly ready to go. I’ve been sitting here thinking and I can’t think of a logical, rational reason to do this. I don’t even see the “cool factor”.
If there is a good reason to do this rather than slap on a zeroed, complete upper, I’m not seeing it.
Did you watch the Video? They claim that it re-zeros with attachment.
More compact storage and carry (like in an unobtrusive attache case) is the big reason. To fit an assembled upper in a similar case, you'd have to go with a pistol-length barrel and deal with the SBR paperwork.