The myth says that the expression “a boot of ale” comes from the custom of cutting off the top of old boots and using them as serving containers. (How the top of an old boot transformed into a vessel is unclear–to me, at least.)
As most of you who work at or visit colonial-era museums know, the American colonists drank out of leather vessels called jacks or blackjacks. These were lined with pitch to make them waterproof and are very sturdy. Decades ago, my stores in Colonial Williamsburg sold reproductions, and I believe they still do. These large leather jugs and mugs made such an impression on the French visitors to England in the 17th century that they reported that Englishmen drank out of their boots! A funny story, not meant to be taken literally. (Waterer’s Leather in Life, Art, and Industry, 1946, London)
Why use leather to make a drinking vessel? It’s an English tradition. In medieval England, there was little glass manufacture, so aside from wood, pottery, or tin, what are you going to use to make a mug or goblet? (Yes, gold and silver, but those are for the nobles, not taverns or average folks, so let’s not go there.) Leather worked very well. Still does. But not boots.