A spiked chain has reach, so you can strike opponents 10 feet away with it. In addition, unlike most other weapons with reach, it can be used against an adjacent foe.
You can make trip attacks with the chain. If you are tripped during your own trip attempt, you can drop the chain to avoid being tripped.
When using a spiked chain, you get a +2 bonus on CMB rolls made to disarm an opponent (including the roll to avoid being disarmed if such an attempt fails).
You can use the Weapon Finesse feat to apply your Dexterity modifier instead of your Strength modifier to attack rolls with a spiked chain sized for you, even though it isn’t a light weapon for you.
Weilding the spiked chain requires the Exotic Weapon feat.