Why do Catholics name their children after saints?

Roman Catholic AnswerThe custom of Christians naming their children after a Saint goes back to the very earliest time of the Church.

from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980

Names, Christian. Names given to individuals at their baptism. The custom of giving the name of a saint when a person is baptized goes back to the earliest days of the church. It is required by ecclesiastical law and means that the saint who name is chosen becomes a special patron to protect and guide and be the heavenly intercessor for the one who bears his or her name.