There is a different number of radiation interactions in your ionization chamber based on the energy of the electrons crossing it (see Bragg-Gray cavity theory). The calibration factor N_D,w^Co-60 is only correct for Co-60. If you don't correct for the different energy of your beam, the charge collected per unit dose you use (the calibration factor) will not be correct and you will calculate the wrong dose.

It mostly accounts for the ratio of stopping powers for secondary electrons of different beam qualities, as well as the ratio of mass-energy-attenuation coefficients in the photon beams generating said secondary electron fluences, as well as a few other chamber-dependent and geometry-dependent correction factors such as the influence of the central electrode, the chamber walls, etc... each of which is energy-dependent.