Diazonium salts are the "Swiss Army knife" of aromatic synthesis. Starting from ArNH2 (easily made from ArNO2), you can access: ArCl $\frac{Sandmeyer}{CuCl}$, ArBr $\frac{Sandmeyer}{CuBr}$, ArI (KI), ArCN -> ArCOOH $\frac{Sandmeyer}{CuCN then hydrolysis}$, ArOH $\frac{H2O}{warm}$, ArF (Balz-Schiemann), ArH (H3PO2), ArNO2 $\frac{NaNO2}{Cu}$, Ar-N=N-Ar' (coupling). Synthetic strategies: (1) To put -F on benzene: aniline -> diazonium -> ArF (Balz-Schiemann). No direct electrophilic fluorination is practical. (2) To put -I on benzene: aniline -> diazonium -> ArI (KI). (3) Meta-directed synthesis: put -NO2 first (meta-director), reduce to -NH2 after substitution, then diazotise and replace as needed.