Why add vodka to batter for frying fish? At 4:46 in this video, Heston Blumenthal adds vodka to the batter for fried fish. This article in Robb Report also describes the process:

The star chef begins his experiment in an elevated way. He’s not using cod or halibut. No, no, no. This is a three-Michelin-star chef, so he’s going right for the whole turbot that he butchers himself to ensure maximum freshness and the correct portion size.
For the batter he mixes flour, rice flour, honey, vodka, and a beer. That’s all pretty standard, until he puts them in a CO2 cannister to make the batter even airier. Once the fish is dredged in flour and coated in batter from the soda cannister, he fries it. For one last step to make the batter crispy and thick, he drizzles more of it onto the fish while it’s in the frying oil.

What’s the purpose of the vodka?
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.