One of the oldest photographic developing agents, having been produced in a pure state by Henry Braconnot, professore of natural history at Nancy, 1831, who found that it rapidly reduced metallic silver from silver nitrate solutions.It was discovered as a developer for paper negatives in 1851 by both Henri Victor Regnault and Justus Liebig simultaneoulsy and independently. It was suggested to be used by Frederick Acott Archer in his wet plate collodion in 1851. It was also used in developing Lippmann Photographs in the 1890's, and remained popular through to the terminal years of chemical photography, and details about that aspect of its utility can be found in the handbook of pyro.
It was popularized as a developer in the Pyrochrome Process.
Skin that turns dark from exposure to pyrogallol can be returned to its original color by immersion in laundry bleach, or an acid permangante solution, or rubbing citric acid crystals between the affected fingers.
The gelatin of a pyrogallol processed hologram looks similar to tanned leather, as that is another use for this chemical.
Here is what the Photo-Lab Index from Morgan & Morgan has to say about it: