Here's why:

* Magnetic field lines: Magnetic field lines represent the direction and strength of a magnet's force. They are concentrated at the poles and spread out further away from the magnet.

* Field strength: The closer the field lines are to each other, the stronger the magnetic field. Since the field lines are most concentrated at the poles, the magnetic field is strongest there.

* Magnetic dipoles: Magnets act like tiny magnetic dipoles, with a north pole and a south pole. These poles are where the magnetic force is most concentrated.

Therefore, if you place a piece of iron near a magnet, it will experience the strongest attractive force when placed near the poles.