Usually said to reiterate the importance of drinking. Just saying "drink" once doesn't convey the sense importance surrounding our drinking culture. Said twice, with some gusto,a party shall evolve, like magic.
Can be compounded by saying "drink" as many times until it sounds something like Drinkdrinkdrkkddir
Person 1 : Yo what do you wanna do today?
Person 2: Drink drink!
Person 3: Yeah man, drinkdrindkdrindk!