I have heard that sphagnum moss has natural rooting hormones in it. Don’t know if it’s true but I sprinkle some into my rooting mixes. I’m happy with my results but I have no way to figure out if the moss aids at all.
Sphagnum is naturally acidic, which is great for both disinfection as well as rooting environment. If it's still alive, it takes up plant sap and processes it to something harmless. This phenolic bleeding of cuttings can be detrimental for root development, so a substrate that counters this is pretty awesome.
I'm a big fan of sphagnum, but the rooting hormone part is
probably untrue. Rooting hormones are pretty 'expensive' for an organism to produce, and it wouldn't be very clever to let those seep out.. Especially for a moss that tends to live in very wet areas where these hormones wash away. I haven't found any clues about this hormone presence in any literature, but I didn't look very well either. Most of the evidence I found is anecdotal and all of said evidence is from bonsai related sources. It seems to be a myth.
Although sphagnum is cool, it can still house gnat larvae, plant-munching nematodes and woodlice. It can keep out a lot of bacteria and fungi, but not critters. Some of my air layers are heavily affected by wood lice that munch the callus tissue and prevent my plants from rooting.