The basic spark plug is made of copper, which is actually a pretty good conductor of voltage. The problem with copper is that it is not as hard as some metals and it therefore wears down with the high pressure and heat inside the engine. This wear eventually causes the plugs to foul and work less efficiently over time.
The wear also can cause the air gap in the plug to increase, which can cause a loss in vehicle fuel efficiency and even misfiring in extreme cases. This is because it takes more and more voltage to jump between the electrodes the greater the air gaps widens. One way around this is to use a harder metal, like platinum.