Sacraments are outward signs of inward grace, instituted by Christ for our sanctification.
The purpose of the sacraments is so that we can encounter God and receive His grace. "Sacraments are 'powers that come forth' from the Body of Christ, which is ever-living and life-giving. They are actions of the Holy Spirit at work in his Body, the Church. They are 'the masterworks of God' in the new and everlasting covenant" (Catechism of the Catholic Church, paragraph 1116).
"Grace is a participation in the life of God" (Catechism, 1997). Jesus came to fill us with His very life, and the sacraments are the preeminent way in which He does this.