Most WH have a glass lined tank with the main tank made out of steel. Steel (mainly iron) rusts. The way a sacrificial anode works is by being more reactive than the material it is trying to protect - IOW, the anode corrodes first because it is more easily attacked, thus protecting the material it is designed to protect. As the anode wears out, whatever it is that attacks the anode now can attack the steel. Primarily, that's dissolved oxygen in the potable water that gets replaced as the tank is refilled from the supply. Now, the glass lining is never perfect, and the heating/cooling as the thing cycles along with any initial imperfections, eventually means that the tank lining will become more compromised and will be attacked, and thus rust out. If you had a sacrificial anode in there all of the time, the tank might end up lasting much longer (things like boats, ship docks, buoys, etc. all tend to have sacrificial anodes attached so that they can last longer). So, what you'll see if you never replace yours as it wears out (most people don't!), is that eventually, the tank will rust out, and leak, and then, you'll have to replace it. If you changed the anode prior to it being used up, the tank would be likely to last longer, which is why they are installed in the first place. In fact, some of the tanks with the longest warranty come with two anodes verses the more common one to help it last longer. Now, if you paid someone to change the anode, over the years of checking and replacing, you'd end up spending about the same amount of money in labor keeping the old one working, verses just leaving it alone and replace it when it leaks. The new ones tend to be both safer and better insulated, and thus often, more efficient. So, keeping the old one working longer may not be really all that cost effective unless you do it yourself and don't value your time and labor (the anode is pretty cheap, but labor to replace it can add up). SO, it's really a cost/benefit decision. The thing will wear out eventually, even if you do change the anode. If you've got the time and the tools (takes a big impact wrench to loosen the thing the first time), feel free to change it. It can give you a few more years of use, but the elements or burner may just wear out in the meantime...those aren't designed to last forever, either.
