The Apostles' Creed is one of the main creeds we use for services. It is present in the services of morning and evening prayer. It's major role, however, is in it's question and answer form in the Baptismal service.
The Apostles' Creed comes from an ancient western formulation of faith. It was meant to be recited by those being baptized as a sign of the faith they were being baptized into.
That is also why the creed comes exclusively in "I believe" form. It is the statement that each of us personally makes as we as individuals are joined in the body of Christ, the church.
Saving the the Apostles' Creed after our baptism is a reminder of our constant renewing of our baptism. Next time you say this creed, stop and think about your baptism, or, if you were baptized as an infant, think about the time when you made your faith your own.