Why do you drink?

Kiddush is not a drink. Kiddush is a ritual and more specifically a pair of blessings that are said over bread and wine at the end of the Sabbath and festival evening and morning services, and also said at the start of the Sabbath and Festival evening and morning meals. (If you're having the meal in the synagogue, you say Kiddush just once after the service and to start of the meal.) The blessings, along with the consumption of bread and wine, are part of the liturgy. Why? A common explanation is that nobody should leave the divine service with an empty stomach.