The reception of the sacrament of Confirmation is necessary for the completion of baptismal grace…
“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.” -CCC, no. 1285, citing LG, no. 11
Confirmation brings an increase and deepening of baptismal grace:
it roots us more deeply in the divine filiation [becoming adopted sons and daughters of God] which makes us cry, “Abba! Father!”
it unites us more firmly to Christ;
it increases the gifts of the Holy Spirit in us;
it renders our bond with the Church more perfect;
it gives us a special strength of the Holy Spirit to spread and defend the faith by word and action as true witnesses of Christ, to confess the name of Christ boldly, and never to be ashamed of the Cross.
-CCC, no. 1303