With electrophilic aromatic substitution (EAS) you use a strong lewis acid to create an electrophile.
$$\ce{AlCl3 + R-X \rightarrow AlCl3X- + R+} $$
$$\mathrm{R = Cl,\ Br,\ I,\ alky,\ aryl,\ RC(O),\ etc...}$$
$$\ce{X = Cl,\ Br,\ I, R-C(O)O,\ etc}...$$
These lewis acids (not the benzene) are incredibly sensitive to water which will irreversably react with the acid to form a hydrate.
$$\ce{ACl3 + X H2O \rightarrow ACl3.X H2O}$$
$$\mathrm{For}\ \ce{AlCl3}: \ \mathrm{1\leq X \leq 9}$$
This hydrate is not as strong of a lewis acid as the anhydrous acid and no longer able to form electrophiles. That is why you must use anhydrous conditions with substitution reactions.