In this reaction ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_3\underset{{\mathrm{CrO}}_3}{\overset{{\left({\mathrm{CH}}_3\mathrm{CO}\right)}_2\mathrm{O}}{\to }}\mathrm{M}\underset{ \mathrm{hydrolysis}}{\overset{\mathrm{alkaline}}{\to }}{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO}$ acetic anhydride is used

If acetic anhydride is not used we will get benzoic acid, due to usage of acetic anhydride the oxidation of benzaldehyde is prevented