Dielectric unions prevent galvanic corrosion.
Updated: Sep 21, 2019
Spend about $10 (during installation) and extend the life of your water heater by preventing galvanic corrosion. Volta invented the battery (dry cell) by putting an acid paste between two different metals. So when you join copper and steal pipes together and run water through them you will get a bit of electric current that deteriorates the pipes and takes calcium salts out of solution. Galvani is the guy who connected Volta's battery to frogs' legs and made them twitch.
The dielectric effect can also be seen in a series of pinhole leaks at he bottom of copper pipes:
Iron particles from a rusty water heater lay along the bottom and corrode the pipe all along its length...yet another argument for replacing water heaters every 15 years! Similar problems near joints can be caused by excess flux inside during soldering and turbulence caused by burrs not smoothed off before soldering.