Probably a mix of old grease and dirt/dust. That stuff will wreck almost everything as it works as a coarse lapping compound. As to why? Possibly corrosion protection.
I know one guy that 'tunes' C&B revolvers and he also pumps the action full of Mobil 1 grease, calling it a 'sealed action'. Keeps the blackpowder fouling out of the action. Can't say I'd try it on my own revolvers.