Anhydrous AICI₃ is used in the Friedel-Craft's reaction because it is

AICI₃ is an electron deficient compound. It generates electrophile in the reaction.

${\mathrm{CH}}_3\mathrm{Cl}+{\mathrm{AlCl}}_3\to \underset{\text{ Electophile }}{{\mathrm{CH}}_3^{+}}+{\mathrm{AlCl}}_4^{-}$