A living filter and a spice rack
Oak is strong enough to hold liquid for decades yet porous enough to breathe — and its wood is packed with flavour compounds: vanillin (vanilla), lactones (coconut), tannins (structure and dryness). Charring the inside caramelises sugars and creates an active carbon layer that strips harshness — the barrel is both spice rack and filter.
Char is a flame burn (bourbon's alligator-skin interior); toast is gentler heat. Deeper char = more caramel and filter; lighter toast = more subtle wood spice.