Bottom line is that a fastener such as a bolt must have the proper elongation within its elastic deformation range to provide the proper clamping force. In our refinery, the mechanical folks use an extensometer in critical service to measure this extension, thus ensuring the proper fastening force is in place. Torquing attempts to do the same but is subject factors that can negatively affect the target fastening force. Some of these factors have already been mentioned...messed up threads, friction between the head of the bolt and the surface that the force is being applied to, etc. Anything that can be done to minimize these friction forces such as lubing threads, cleaning threads, etc. will help ensure the torque method provides adequate fastening force. Hope this wasn't too technical.
