Why does the priest put water in the wine at Mass?

The practice of mixing water and wine was common in the ancient world. Wines were usually heavier than most modern vintages, and to dilute them a bit made them more palatable and less inebriating. People also drank more wine since water in the ancient world could not be purified easily as is done today. Thus, the wine used at Mass was mixed with water before the consecration in the usual manner of all wine. 
 
Mystically it came to represent our inclusion into Christ's Body by our baptism. The priest says: "By the mystery of the water and wine, may we come to share in the divinity of Christ who humbled himself to share in our humanity". 
 
Though the practical reason to mix water and wine no longer is needed, it still remains a powerful symbol and so its practice remains.