Pop cans, or soda cans, are typically treated with a plastic coating that protects the aluminum material of the metal container from the potentially corrosive soda drink that is held within.
Perhaps you have seen the popular chemistry experiment that demonstrates how cola can dissolve a tooth or clean the rust off of a coin. This is because many soft drinks are highly acidic and can react chemically with many types of metals. Since aluminum is what pop cans are typically made out of, the plastic lining of the can keeps the liquid from harming the material it comes in.
It if weren't for the plastic coating inside of these soda cans, chances are your can of pop would have rotted away before it even made it to your refrigerator. Another consideration is that if your can were to become corroded by the soda within, pieces of aluminum would chemically combine with your drink and make the liquid unsafe to consume.
Originally, soda was packaged in tin cans, but manufacturers switched to aluminum because of its lighter weight and durability. Plastic coating was a more recent innovation that extended this durability further.