My pyro notes from a private class 16 February 2008 with Per Volquartz.
Per looked at my Tri-X 400 120 negatives which I had developed in stock XTOL and said that if I am happy with the grain and sharpness, that I should try pyro for the following reasons:
hardens the gelatin
stains the gelatin
continuous grain for fog, glass, skin, water
selflimiting - compresses range
keeps highlights by preventing the highlights from burning out

I have since moved to XTOL replenished. I have the pyro and TF-4 but I have not mixed it up yet.