"Shear nuts have 1/2 the thickness of regular nuts and are used in shear rather than in tension. Often the torque requirements of fasteners used in shear are quite different. Generally, the torque values for shear bolts and nuts are 1/2 the values in standard torque tables."
-per "NASA Reference Publication 1228, Fastener Design Manual"
For use in Bearing Critical shear joints
Shear nuts are used in bearing-critical joints. A bearing-critical joint is one in which the bolt takes the shear load. The bolt acts as a pin and keeps the joints from sliding. Bolt torque needs only be enough to bring the joints together and to prevent nut back-off. Due to the low torque value, lower strength (shear) nuts can be used.
Shear-critical depend upon joint friction to resist shear loading. Friction keeps the joints from sliding. They are friction dependent. In shear-critical joints the bolt needs to be tightened more so that the clamping forces produce high friction so the joint doesn't move. Sufficient joint clamping power to prevent joint movement determines what bolt, nut, and torque to use. Because of the higher torque values, a full strength nut is usually required.
Shear-critical joints are often found in aircraft where vibrations and load reversals would move the bolt in the hole and create wear. Shear joints are also needed to improve joint durability and fatigue resistance.
A common example of the difference between bearing and shear-critical is on old houses with hardwood floors. Nails hold the planks in place. As people walk on the planks, slight movement, over time, enlarged the nail holes so that the nail became slightly loose in the hole. The floor squeaks as the plank moves. Modern hardwood floors are glued. The glue keeps the plank from moving and a shear-critical joint is made between the plank and the floor. No squeaking.