Why do Catholic Christians make the sign of the cross over their bodies or trace the sign of the cross on their foreheads, often referred to as signing or blessing oneself? Is it merely ritualistic? Is it superstitious? What's the point?
Many things could be said about this
ancient prayer devotion. It's roots find themselves deep in history's soil: back to the early centuries of Christianity, in fact. For the time being, however, I will have to limit myself to just a few points.

First of all I'd like to draw attention to the sign itself. It is, of course, of a cross, and precisely the cross of Christ. Every time it is made, this sign is meant to bring our thoughts back, from wherever they are, to the central act of love made in human history in the Crucifixion of our Lord Jesus. What greater love is there than that of a man laying down his life for his friends (and, I might add, enemies)?
Secondly, while making the sign of the cross with one's hand, he prays, "In the name of the Father, and of the Son, and of the Holy Spirit." This is not merely a dry or empty formula, some kind of magical pattern, like abracadabra. But rather it is an invoking of God, Who is One in three persons, to bless us. These words, all by themselves, are a profound prayer calling upon God to hear us and bless us in and through the redeeming act of love Christ Jesus made for us (represented in the sign). (It is also a summary of our faith-a kind of seed version of the Apostles Creed which all Christians ought to be able to confess). And so, to make the sign of the cross, even if the words are not spoken out loud, like any prayer, is to pray for God to come to our aid. And it is a Christo-centric prayer reminding us of and keeping our focus on the essence of our Christian faith. As the apostle Paul said, "We preach Christ crucified." (1 Cor. 1:23)
As a side note, because we are made of matter and have senses that interact with the material world around us, physical things and signs can be useful to us. They can serve to remind us of spiritual realities or real historical events that are important for us to have in front of us (as in the case of monuments). Indeed, this is what God has done in His incarnation, He manifested Himself (Spirit-ual) to us in human (material) form. The God-Man, Jesus, is then, the icon, the image, the sign of God among us. When we make the sign of the Cross, we make these realities present to ourselves once again.
For a more thorough list of things 'we're doing' when we make the sign of the cross, click
here.