Why is sodium bisulfite needed in syn-dihydroxylation?

On the reaction of syn-dihydroxylation with stoichiometric quantities of OsO4 (not a catalytic amount) the second step is $\ce{NaHSO3/H2O}$ or $\ce{Na2SO3/H2O}$.

According to my understanding, $\ce{H2O}$ is the only thing needed to hydrolyze the cyclic osmate ester resulting from the first step of adding $\ce{OsO4}$.

So, what is the use of $\ce{NaHSO3}$ or $\ce{Na2SO3}$ here?