Automobiles still require maintenance and this includes spark plugs. Even though your vehicle may be equipped with long-life platinum plugs, having them changed before failure will likely save you money in the long run. Misfires that occur in your engine can wreak havoc on your catalytic converter(s) which can leak to big $$$$s!
This short video demonstrates how. Spark plug can be tested on the bench and more importantly shows one of the failures a spark plug can experience and where the high voltage used to fire the air fuel mixture can occur in an undesirable location.