Question

Electrode potential of Standard hydrogen electrode is 0. Still is used as a primary referance electrode. justify.

Answer 1

It's the same reason why 0 on the Celsius scale coincides with the melting point of water - we need a common baseline against which we can compare the temperature of other things. So in electrochemistry we give the hydrogen electrode a zero potential so that we have an objective baseline against which to compare all other electrodes. SHE is a Reference electrode that is why its electrode potential is taken as zero at standard conditions(at 298 K temp, 1atm pressure and at 1M concentration of HCl).