Most B&W developers contain Hydroquinone with either Metol or Phenidone or a Phenidone derivative. Both Metol and Phenidone are super-additive with Hydroquinone, so why add both of them in the same developer formula?

Here is an example of a developer which contains all three of those constituents.
http://www.tetenal.com/sidaopener_uk...auk/100152.pdf