The inside of oak whiskey barrels is charred for 2 reasons. One is that the charring and heat applied to the oak brings out flavor notes in the liquor, but the other seems to be just a result of the old manufacturing process (coopering) where they would start and put the barrel around it while assembling which I assume was to create a steam bending conducive environment. Pretty neat happy accident that the way the barrel is made basically invented certain flavors of booze.