Refer to figure.
Centre of Pressure (CP) is the point on the chord line where the resultant of all the aerodynamic forces acts.

On a symmetrical airfoil, the position of the CP remains relatively unchanged at about 25% of the chord line with the change in Angle of Attack, provided that the angle of attack remains within its normal operating limits (not beyond the stalling angle of attack).

Since the position of the CP is more or less fixed, symmetrical airfoils are useful on helicopters, because the variation of the pitching (twisting) moments and extra loads on the control links are small.

Pitching moments depend only on the shape of the mean line between the two surfaces. If the mean line is straight, as with a symmetrical airfoil, the moment will be zero.

So, if a blade with a symmetrical profile has its feathering axis lined up with the quarter-chord, it should feel no aerodynamic pitching moment in flight. If there are large changes in pitching moment, the pitch control system can be easily overstressed.

Consequently, most rotor blades sections are designed to produce the least variation in CP with angle of attack and to achieve good torsional stiffness, with the blade balanced, so that the Centre of Gravity is coincident on the chord line with the CP, Aerodynamic Centre and feathering axis of the blade. Torsional stiffness can be more easily achieved when using symmetrical airfoils.