WHY DO ABSOLUTE VALUE OF ELECTRODE …

It is because  practically we fail to determine electrode  potential of a half cell consisting of an electrode dipped in its own salt solution. We, can measure only the difference between two half - cell potentials. that gives emf of the cell . If we arbitrarily choose the potential of one of the electrode ( half cell ) then  that of the other can be determined  with respect to this.  Therefore  ,   we need to set up a galvanic cell  consisting of two half cells , and measuring  out its  EMF.  This is possible only when we construct a cell with two half cells  & calculate its  EMF  (  in volts ) using the expression

E^o _cell  =  E^o_R  -  E^o_L 

Hence ,   electrode potential  of  one of the half cells  must be  known .  As per IUPAC norms  Hydrogen electrode  has been chosen  under standard conditions (ie. H₂   with  1 bar  press . ,[ H⁺ ]  =  1 M  &   at 298 K  )  with its arbitrarily fixed electrode potential as   equal to zero.

So , the reduction electrode potentials of all the different electrodes have been determined  and reported as electrode potential  relative to that of  hydrogen electrode .