That's just a recommendation based on someone's estimate of how long it takes on average for a chain to wear out. Wearing out means that it streches to the point that the chain links no longer line up properly with the gear teeth on your bike, leading to excessive wear on the cassette and chainring.

Check out Sholdon Brown's site for info on how to measure chain wear. If your chain has not stretched beyond acceptable limits, it does not need to be replaced regardless of how many miles it has. Campy chains are both more expensive and longer lasting than some other brands so you don't want to toss it out too soon,