A torque wrench is designed to tighten stuff and to the correct tolerances, measured in Newton Metres. Most modern components - bars, stems, seat collars, pedals, seatpost binder bolts, saddle rails commonly have recommended settings e.g. 5, 10, 12nm either etched on them or detailed in their instructions. Too much, or too little torque can damage them, which isn’t always obvious. Another reason why gripper pastes are used on carbon components so you can go to the lower end of the recommended setting without risk of damage, or over-stressing. A click-type torque wrench will have a range and when you've hit the desired number it will let you know, often with an audible click, going limp, or a combination of both. 

However, they require an element of mechanical sensitivity, you can exceed the pre-set torque by tightening beyond the first click or going momentarily limp - running a very real risk of fatigue, or immediate breakage. The other point is that torque wrenches are designed for tightening, not loosening stuff-doing so runs the very real risk of damaging the tools precision mechanism. 

Expensive models can be repaired and recalibrated, but it's easily avoided. Always store in a solid box and somewhere they won't fall or get dropped since this to can damage calibration and accuracy. Oh yes, always set to zero, or minimum tension when you've finished fettling.