Generally, in the majority of applications, the reliability of the joint is dependent upon the bolts ability to clamp the parts together.
From Mountz' Torque Tips Page
Adequate clamping prevents relative motion between parts of the joint and leakage from joints containing gaskets. Measuring a bolts clamp force is difficult, especially under production assembly conditions.
The clamp force generated by a bolt can be indirectly controlled by regulating the applied torque. This method, known as torque control, is by far the most popular method of controlling a bolts clamp force. The initial clamp force generated by the bolt is frequently called preload.
There is a link between the torque applied to a bolt and the resulting preload. A problem exists in that friction has a large influence on how much torque is converted into preload. Besides the torque required to stretch the bolt, torque is also required to overcome friction in the threads and under the nut face. Typically, 10% to 15% of the torque is used to stretch the bolt. Of the remaining torque, typically 30% is dissipated in the threads and 50% to 55% under the nut face. Because friction is an important factor in the relationship between torque and preload, variations in friction have a significant influence on the bolts preload. Different bolt surface finishes generally have different friction values.
The torque required for a socket head screw will not be the same as that required the same size standard hexagon bolt. The larger bearing face of the standard bolt will result in an increased torque being required compared to a socket headed screw. This is because more torque is being dissipated between the nut face and the joint surface.
Read the rest from Mountz, The Torque Tool Specialists.