why oixidation reduction takes place at standard hydrogen electrode?

The standard hydrogen electrode is a half-cell used as a reference electrode. In the half-cell, hydrogen gas (1 atm) is bubbled through an acid solution in which [H₃O⁺(aq)] = 1 mol dm⁻³. Platinized platinum is used as the electrode.

In a complete circuit containing a standard hydrogen electrode and a half-cell with positive reduction potential, oxidation occurs in the standard hydrogen electrode :
H₂(g) + 2H₂O(l) → 2H₃O⁺(aq) + 2e⁻

In a complete circuit containing a standard hydrogen electrode and a half-cell with negative reduction potential, reduction occurs in the standard hydrogen electrode :
2H₃O⁺(aq) + 2e⁻ → H₂(g) + 2H₂O(l)

Hence, an oxidation reduction equilibrium is established in the standard hydrogen electrode.
2H₃O⁺(aq) + 2e⁻ ⇌ H₂(g) + 2H₂O(l)