Why do we use megger to measure earth resistivity although this value is very small?

MEGGER IS use to apply  high amount of voltages to soil to chek restivity. whene any fault occure the large amont of current flow through earth pit to soil.. so its depend on soil restivity.. 

To Provide good grounding with almost 0  resistivity.. ( to take current easy path whenvever faulty condition appears)

To verify the condition of Electrical insulation in minimum level of resistance, by applying selected voltages.

To check the insulation resistance of the cable from which we can understand the cable is healthy and it is applicable to energise 

it is capable of measuring the flow of resistant thru the earth in which can provide a small value or zero current and be called a direct line current

This device enable us to measure electrical leakage in wire. This device is basically use for verifying the electrical insulation level of any device like motors, cables and generator winding.