Over time, distributor caps that still look good, are supposed to build "carbon trails" inside, that can "re-route" spark from the right terminal to another terminal on the cap (fires the wrong cylinder) causing a misfire.
Frankly, I think the caps will last a lot longer than the "scheduled maintanance" calls for, (Similar to the to the idea that changing your oil every 3,000 miles is a good idea, I think this is largely a waste of money,,,)
But then again what does a cap cost, like $10.00?