With this practice we achieve a number of things. We establish the rite of libation, we train ourselves to think of food as a blessing, we are reminded of the many layered nature of reality. And it begins the long process of developing the purity which prayer provides. But though alcohol is a traditional medium of libation, the matter goes further than that. Which of several types of liquids you pour out has its own meaning.
Wine represents the blood of the earth and is a suitable offering for almost any purpose.
Whites as well as reds, by the way, have this meaning. Beers represent oath and community. They can be poured to strengthen friendship, to give or receive an oath, or to form a compact or agreement. Milk is a symbol of family, kindness, and acceptance. Such libations can be poured from any cup, bowl or dish, though in ancient times special vessels were often created for libations.