Why is sulfuric acid used in crystallization
The sulfuric acid will dissolve any residual copper metal or any copper oxides that exist as impurities in the copper sulfate, converting those impurities into copper sulfate.

$$\ce{H2SO4(aq) + Cu(s) -&gt; CuSO4(aq) + H2(g)}$$
$$\ce{H2SO4(aq) + CuO(s) -&gt; CuSO4(aq) + H2O(l)}$$
$$\ce{2H2SO4(aq) + Cu2O(s) -&gt; 2CuSO4(aq) + H2O(l) +H2(g)}$$