Hits Normal, Rock, Ice, Dark and Steel — the best offensive coverage in the game.
Fighting-type moves are super-effective against five types — more than any other single type. That is the reason Fighting has been in every competitive metagame since Gen 1: no matter what's popular, Fighting hits something important for double damage. Its signature weakness — Psychic and Fairy — is also its biggest meta problem, since Fairy's introduction in Gen 6 specifically nerfed the type.
Fighting is a physical-attacker type. Almost every viable Fighting mon runs Close Combat or a similar 120 BP STAB move. Pairing Fighting with Flying or Psychic coverage checks both of its main counters. Bulk Up setups and Choice Scarf sets are the two dominant archetypes.
Close Combat (120 BP, drops Def/SpD) is the nuke. Drain Punch (75 BP + healing) is the bulk-up set's recovery. Aura Sphere never misses. Mach Punch gives priority for revenge kills.
Machamp's four arms were explicitly designed by Ken Sugimori to make it 'look like it could hit you from any angle'. In the anime, Machamp characters canonically use all four arms independently to multitask.