For when you want to know what you are actually agreeing to. It is what the whole church stands up and says together every Sunday, and it was written by two councils sixteen centuries ago to settle a fight — and then one word was added in the West that started another.
It is not a prayer in the asking sense. It asks nothing. It is a list of what you believe, said out loud in front of other people who are saying it too, which is a different act — closer to a promise than a request. That is why it is said standing.
Every clause in it was fought over, and the shape shows it. God of God, Light of Light, true God of true God, begotten, not made, consubstantial with the Father — four ways of saying one thing, because in 325 there were bishops who would sign three of them and not the fourth. The repetition is scar tissue.
Two councils. Nicaea in 325, called by Constantine, produced the first version to settle whether the Son was God or a creature — Arius said creature, and the word consubstantial was chosen precisely because he could not say it. Constantinople in 381 expanded the last section, about the Holy Spirit, and that is the text said at Mass.
Then one word. The Latin West, starting in Spain in 589, added and the Son — Filioque — to the clause about where the Spirit proceeds from. Rome adopted it in 1014. The Greek East never did, and in 1054 the two halves of the Church excommunicated each other; the word was not the only reason, but it was the one on the table. Every Sunday you say the clause the East still will not.
It is the one creed shared by Catholics, Orthodox and most Protestants, one word apart. It was sung in Latin at Mass for a thousand years, so composers set it, which is why the Credo is the longest movement of every great Mass setting — Bach's, Beethoven's, Mozart's.
In English it has been through three versions in fifty years: the 1973 We believe… one in being with the Father, and the 2011 return to I believe… consubstantial, which is what the traditional English above always said. The shorter Apostles' Creed is the one said in the Rosary.