Just to reinforce what megan has stated, the nuts and bolts are tightened to a design torque as specified by the manufacturer of the aircraft. The safety wire is then inserted to guarantee that the specified torque is maintained, i.e. the nut and bolt cannot slacken due to vibration or other influences such as temperature changes. The safety wire is the only indication that the nut and bolt, turnbuckle or whatever fastening is at the correct tension.
So while technically a correctly tensioned nut and bolt are theoretically capable of withstanding the physical aerodynamic stresses imposed, the slightest turn without a safety wire in place will place that connection outside the design parameters. It is then unsafe.
A browse through the air accident investigation archives will reveal many accidents caused by missing locking wires, particularly relating to engine components on light aircraft.
Vibration and thermal cycling are the main causes of threaded connections coming undone. Both are present in all aircraft during flight.