The function of , in Friedel-Craft reaction, is to produce electrophile, which later adds to benzene nucleus. This electrophilic aromatic substitution allows the synthesis of monoacylated products from the reaction between arenes and acyl chlorides or anhydrides. The products are deactivated and do not undergo a second substitution. Normally, a stoichiometric amount of the Lewis acid catalyst is required for both the substrate and the product form complexes.
(and other Lewis acids like it) will coordinate to halogens, and facilitate the breaking of these bonds. In doing so, it increases the electrophilicity of its binding partner, making it much more reactive.