Why are concentric springs fitted for A/E cylinder head valves?

If the frequency of the natural vibration of air or exhaust valve springs is a harmonic of the 
camshaft speed, then the spring may vibrate axially and are said to surge. Surge can be 
avoided by modifying the sizes of springs by arranging the springs in pairs and fitting one 
inside the other. 
If one spring fails the valve is held up and is not damaged by striking the piston. 
The thickness of individual springs can be reduced as against original thickness. 
Normally two springs of different vibration characteristics are chosen. 
Moreover, the stiffness requirements and the space congestion at this place warrants a 
concentric springs arrangement. The net stiffness is equal to the sum of individual stiffness.