KMnO₄ can be prepared from K₂MnO₄ as per the reaction: 
$3{\mathrm{MnO}}_4^{2-}+2{\mathrm{H}}_2\mathrm{O}\rightleftharpoons 2{\mathrm{MnO}}_4^{-}+{\mathrm{MnO}}_2+4{\mathrm{OH}}^{-}$
The reaction can go to completion by removing OH^- ions by adding.

HCI and SO₂ are reducing agents and can reduce ${\mathrm{MnO}}_4^{-}\cdot {\mathrm{CO}}_2$ which is neither oxidising and nor reducing. Therefore, CO₂ is added to the solution as it helps to shift the equilibrium in the forward direction by successfully removing hydroxide ion and reaction can go to completion.

${\mathrm{CO}}_2+{\mathrm{OH}}^{-}\to {\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}$