You're right in that once things are tight, additional torque won't change things. But what's "tight"? There are 9 parts in this assembly: hub, two bearings, spacer, companion flange, stub axle, nut, and washer. Bearings are being pulled into their seats in the hub, the spacer is being loaded, a washer and the new stub axle nut are being tightened together for the first time, the stub axle is being stretched, etc. It may take 180 ft. lbs. to make things tight in one assembly and 240 ft. lbs. in another.