[While John does carry it around just in case COMPASS decides to snatch him away again, it also holds things he might need here in case of emergency. If he comes across an individual who needs medical attention while he's out and about, he can treat them. It also means he can put his gun in it or even put in other items that he might pick up.]

I've heard some of it, never seen it for myself. Honestly, I imagine it's extremely painful to go through the transformation between the body he has now and the one he should have.