Inner race of the wheel bearing is made of two halves, which are held together by the hub (outer half) and the axle (inner half). That big bolt in the axle's center actually holds the bearing halves together. If you remove it (and the axle), then the bearing's inner half of the inner race (I hope that still makes sense) is free to walk off the hub.

If you're lucky, you can just bolt everything back together with no damage. If you're less lucky, you can score some surfaces & bearings will eventually fail. If you're extremely unlucky - your wheels can fall off.
Hope this answers your question.