As the normal HCl (aqueous hhydrochloric acid) available for laboratory puproses is a mixture of Hydrogen chloride+water, even the concentrated HCl is available at various concentrations also contains water. Some of the organic reactions which uses HCl might be sensitive to water, so for such type of reactions we need HCl free from water, so we will use either methanolic or ethanolic or alcoholic HCl for such type of reactions which are completely free from water. So adding acetyl chloride either with methanol or ethanol will give us the required form of HCl for the reactions to undergo under water-free conditions.