The torque requirement equates to tension on the screw. When you tighten a screw or bolt, the bolt stretches slightly(gets longer). Think of it as a spring. A bolt might have a torque requirement that will create a tension on the bolt called the proof load. Any more torque, and the bolt will permanently stretch and will essentially not be usable. (There are applications where this is actually desired. "Stretch Bolts". ). Think of bending a metal spoon. You can bend it slightly, and it will return to its initial position. Bend it more and it will be permanently bent. (You exceeded the "yield" point of the metal.) Typically, the torque and hence tension will be in the elastic region( the screw will not be permanently stretch ed and can be reused). Anyway, the tension on the bolt clamps the items together and the frictional force created between the two parts is what prevents the parts from moving. More torque, equates to more load which equates to more clamping strength.