Specific stiffness is a way of comparing materials of different density and different elastic modulus to find the most effective material. The specific stiffness is calculated as E-modulus / density.
In this comparison aluminium is equal to steel, as the aluminiums lower stiffness is compensated by that it has a lower density. The CF composite showing a high E-modulus and a low density outperforms both.
This is especially important in moving machine parts, where stiffness is needed for accuracy of the moment and the low weight allos for fast movement and accelerations.