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@benrb
Shredder is spot on here. You tighten a bolt to a certain level to avoid it coming undone or to create a clamping force. What you are actually trying to achieve is a certain level of bolt elongation. Torque is the most easily measurable indicator of this but it isn't very accurate. It can be effected by simple factors such as lubrication, material or surface finish of the fasteners. So the torque wrench can measure torque very accurately but not bolt tension.
However doing it by feel is susceptible to the same factors.
Using a torque wrench can really help when you're not sure just how tight something should be.