Why do Catholics make the Sign of the Cross?

Catholics are not the only religion that uses the Sign of the Cross. Most Anglicans, Orthodox, Lutherans and some other Protestant denominations use it. However, Catholics are the only ones who routinely use it outside of Church services in personal prayer. It is a form of prayer especially when the words 'In the name of the Father, Son and Holy Spirit' are said while making the sign.
I believe the four points of the cross they make are supposed to stand for "father, son, holy, ghost"

Roman Catholic Answer

The sign of the cross is a confession of faith in Christ crucified and an invocation of his blessing made by a cruciform gesture on one's body... The occasions upon which the action is directed or recommended are innumerable . . . No magic power attaches to the sign of the cross; the action must correspond to a devoutness of heart and mind or it is a useless observance; this does not mean that the mind must necessarily and always advert to the action: it may well be done unconsciously by one habitually religious. from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980

St. Cyril of Jerusalem (315-386 A.D.:Be not ashamed of the cross of Christ . . . but openly seal it upon your forehead, that the devils may behold the royal sign and flee trembling far away. Make this sign at eating and drinking, at sitting, at lying down, at rising up, at speaking, at working.