Why add vodka to batter for frying fish?
According to this article about Blumenthal's method, which also explains the other ingredient/method choices: https://www.nytimes.com/2007/03/07/dining/07curious.html

The key to the Fat Duck batter is the alcohol, which does a couple of
very useful things. It dissolves some of the gluten proteins in the
wheat flour, so no elastic network forms and the crust doesn’t get
tough. (You’ll notice when you combine the ingredients that the mix
becomes mushy rather than sticky.) Alcohol also reduces the amount of
water that the starch granules can absorb, and boils off faster than
water, so the batter dries out, crisps and browns quickly, before the
delicate fish inside overcooks. The crispness lasts through the meal,
and revives well the next day in a hot oven.