It prepares pure $1^\circ$ aliphatic amines because the intermediate N-alkylphthalimide is formed via an $S_N2$ reaction on an alkyl halide, avoiding over-alkylation. It fails for aromatic amines because aryl halides ( $Ar-X$ ) do not readily undergo nucleophilic substitution ( $S_N2$ ) with the bulky phthalimide anion under typical conditions due to partial double bond character and electronic repulsion.