Tanning and tannins share a common root.
Acorns are high in tannins, which, among other things, make foods astringent (painfully chalky). Repeated soakings in water reduces them and you can make a flour.
But now you have a bunch of water high in tannins, which work as a preservative. I'm not sure how someone figured out you could soak leather in it. But I know that walnut colored stain can be made from walnut husks, so decoration may have lead to observation of improved durability. If it works for walnuts why not acorns?
I think this is why pottery was such an important step in human technology. After pottery, the process of boiling the shit out of things to make them palatable, safe to eat, or useful for crafts expanded our resources tremendously. There are cultures that simmered things without ceramics but it's complicated work to keep from damaging your container. Imagine doing all the detailed work to weave a water-tight basket and then your child manages to burn a hole into the damn thing.