What does the Church teach?
"By the Sacrament of Confirmation, [the baptised] are more perfectly bound to the Church and are enriched with a special strength of the Holy Spirit. Hence they are, as true witnesses of Christ, more strictly obliged to spread and defend the faith by word and deed." C.C.C. 1285, quoting the Introduction to the Rite of Confirmation in the Roman Ritual.
Confirmation is the Sacrament that is necessary for the completion of Baptismal grace because it transforms us, God's adopted children, into adult witnesses of God (C.C.C. 1285). It is the Sacrament that marks a stage of spiritual maturity: our way of life changes as our understanding deepens. Just as children do not take responsibility for their own lives, for their feeding, security, education etc. but rely on adults to arrange this for them, so too, while we are spiritual children, our spiritual lives are more passive than active. We are trained in the mysteries of the faith, we are helped to develop and deepen our experience of Christian prayer through parents, friends, priests and others. In this stage, we are being fed, and it is others who take responsibility for feeding us. However, the time comes when we grow up sufficiently to take responsibility for our lives. As teenagers, we begin to decide for ourselves what we will eat, how we will work, whether we will apply ourselves, what we will read and learn. We take responsibility for the decisions we take: if we don't do the work we are expected to do, we are answerable for it - and may end up working through the night in order to meet a deadline. This marks a wholly different attitude within us: no more are we children with all things arranged for us - now we are more responsible for ourselves and are able to identify what we need to do. Most importantly, we have the maturity to organise this for ourselves. Confirmation marks this change and gives us the grace to make the same change in our lives of faith.
NOTE, though, that Confirmation is not just a ceremony of passage, a liturgical way of saying "Spiritually, I have come of age" - it is the moment when God gives us the necessary gift which enables us to come of age. The gift of the Holy Spirit in Confirmation turns a child-like witness of the death and resurrection of the Lord into an adult and fully-activated witness: now we will be able to fulfil the commission of our Baptism which is to bear steadfast and convinced witness to Christ throughout the world. In Confirmation, God provides the strength and divine life that is necessary for us to be able to fulfil our baptismal calling - thus it is truly sacramental (an outward sign which conveys an inward grace to the soul through which the Father gives us his Son).