The decision on whether or not a anode rod is required/needed is based on the
Galvanic Table of Compatibility for dissimilar metals. The further apart the metals are in the table the more they react with each other.
Really the above is not relevant to your question, but I thought it might help better understand why an anode rod is not needed in some WH tanks. If your tank has an anode rod, the tank should be flushed out every year because the metal that flakes off the anode rod settles in the tank bottom.
If your tank doesn't have an anode rod, it is for one of two reasons. One, isn't supposed to have one. Two someone (previous owner) threw it away and just used an iron pipe plug.