The Sacrament of Confirmation is necessary for the completion of baptismal grace.

Question: Why is that? Isn't the Sacrament of Baptism enough? Doesn't it provide all of God and his gifts, since we're baptized in the Father and the Son and the Holy Spirit?

Answer from the Catechism of the Catholic Church, paragraph 1285:

By the sacrament of Confirmation, [the baptized] 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.