A sacrificial anode is one of those little things that mean a lot. They keep steel boats, marine engines, water heaters, bridge pilings and many other wet metal objects from rusting into dust prematurely.
You can see what a sacrificial anode looks like in a marine engine in this video:
You can see the sacrificial anode for a water heater in this video:
Why do you need a sacrificial anode? Because, by using an anode made of a metal like zinc, the anode will decompose before the steel will rust, so the steel is protected. Here's a very nice discussion of why iron and steel rust, and how a sacrificial anode stops the corrosion process:
Even metal caskets use sacrificial anodes. Learn more and see the anodes installed in this video: Some Assembly Required: Zinc Bars for Caskets