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It has to do with the coefficient of friction. The axle drives the hub through the key, which is exposed to shear force when torque is applied to the axle shaft. The reason for a dry fit, hub to axle, is that the hub is driven up on the taper when the axle nut is tightened, to somewhere around 200 ft/lbs.. The friction is needed to aid in keeping the axle from shearing the key. If the axle/hub was lubed, then friction drops, there is more stress on the key, and a potential for failure. Some people will check the axle/hub fit with machinist blue to confirm there is good contact on the surface area. Don't forget there is a seal (looks like a fiber washer) that fits in the outside end of the hub, before the washer and nut go on.