Why Brake Shoes Need to Be Changed
With time the friction material of the shoes wears away, and periodically they need to be replaced. The brake shoe is meant to be the wear item in the braking system. It is softer than the iron drum so that the inner surface of the drum won't wear away. If left too long, the shoes will wear unevenly until the metal backing or the rivets holding the friction material start to contact the drum causing a horrible noise and wear to the drum.
The brake shoes can also become contaminated with grease or oil, from a wheel bearing failure for instance. Accidentally driving with the parking brake on can also glaze or overheat the surface of the shoe, necessitating a change, as can hauling a heavy load or towing.