Alcoholics need their daily top up...
Joking. It's an alternative to distilled water for the purpose of diluting thicker juices, distilled water lessens the throat hit but vodka doesn't. I think it's for people who are PG sensitive. I read it also helps getting flavours to "pop" but to be fair I've never used it myself, this is all from my research so maybe someone with experience will correct me if I'm wrong.