​​Why we put Salt and Charcoal into Earth-pit while Earthing?

Earthing resistance is not the resistance of the electrode. The earthing resistance is not same all over the area. The value of earting resistance is depends upon the soil resistivity of the particular area. Actually the measured earth resistance is not only the resistance of the electrode, it is the combined resistance of conductor and the associated soil. During fault, if the soil resistance is higher the fault current cannot dissipate to associate soil.
To improve the conductivity of poor soil during fault current, salt and charcoal around the conductor is used to dissipate. 
Charcoal is used to retain the moisture for long duration because it is an adsorbent and salt is added to increase the conductivity both these products help to pass the leakage current through earth wire as early as possible to remove the chances of shock.
We can practice permanent earthing compounds which are moisture independent as per IEEE 80–2013 clause 14.5(d).