The distributor cap and rotor, also known as the ignition cap and rotor are main components of the ignition system. The voltage that is created by the ignition coil travels to the ignition cap through the coil to the rotor. As the ignition rotor turns in the distributor, the rotor distributes the spark by delivering it to the spark plugs via the spark plug wires at exactly the right time.
Because the voltage that is created is considered high voltage, it is important that the components are well insulated. If the insulation breaks down, then the spark is not delivered and the engine will run poorly, or not at all. If the cap shows signs of wear, then it will need to be replaced possibly along with the rotor.
Most manufacturers recommend that the distributor cap and rotor be inspected and/or replaced every 30,000. To be sure. refer to your owner's manual for the specific manufacturer recommendations. If the cap and rotor go bad, most manufacturer factory warranties and extended auto warranties will cover the cost of a replacement.