More specifically modern tapered bearings are designed to run under continuous preload (the shaft inside with a nut provides constant clamping force so that the bearing is constantly in contact with its entire bearing race). These older bearings run with no preload, I think largely because machining tolerances were poorer and the races were not always perfect, so the weight of the car applies load to force bearing contact only where needed. The downside is that these bearings need repacking with grease and adjusting very frequently.