Yup, they stop your tank from rusting out. If you replace them regularly your tank will last virtually forever. If you don’t, you’ll be replacing the water tank before too long. A sacrificial anode costs around $50 bucks or so, a tank costs a lot more.

Here’s a video showing you how to do it. It’s not very hard at all. https://www.thisoldhouse.com/heating-cooling-2/how-to-replace-an-anode-rod-on-a-water-heater