If your lugs strip, and say your tire is flat, rim damaged, and the rim is NOT coming off no matter what you think you can do. So, how is the rim coming off? Well, with the center hole, they can remove the HUB with the rim attached, and replace it, so you can get mobile again. They may be able to save the unmangled rim by punching the studs out from the other side too.
Then, there is also a lip around the center hole, it helps center the tire for being mounted back to the hub. The rim can't be filled in at center, because the hub center is raised, lipped, to fit inside the center hole. Ever wonder how much harder it would be to bolt the rim on yourself if there was nothing there to hold it center?
Why is it REALLY there? Probably more than just those two reasons; a place to grab the rim when it's molded, place to hang it to cool off, and whatever MFR sequence they go through.
The Technical: Hub centric wheels have a hole at their center that fits closely over a round feature on the hub, which centers the wheel on the axis of the spindle, as well as bear the vertical weight of the vehicle. The bolts (lug nuts) then only have to secure the wheel onto the hub and are loaded only in tension where they are strong. Factory wheels are all manufactured to fit their specific application exactly, and even some aftermarket wheels as well.
