The Eucharist is the sacrament that preserves the souls' union with God and fosters that union by making a person more holy especially in the practice of the supernatural virtue of charity. As a sacrament of the living, to obtain the graces intended, a person must be in the state of God's friendship when receiving, otherwise the reception becomes a sacrilege (I Corinthians 11:27-29).
The union of the communicant with Christ in the Eucharist is effective in the moral order. Though physically present in the communicant, Christ is not physically united with him. Only the consecrated species, since they alone can come in contact with material things, are physically united with the communicant.
Communion aims specifically at producing a likeness to Christ in the communicants. Their acts of mind and will, as a result of Communion, are to become more conformable to the acts of Christ's mind and will. Their body, too, is to become more like Christ's sacred body.
This is the primary purpose of the sacrament, a special union of the soul with Christ. What is special about this union is that the Eucharist is extraordinarily powerful in conferring actual graces that prompt a person to make acts of love for God and one's neighbor. Moreover, these graces inspire one to live for Christ habitually, even under great difficulties, as shown by the readiness to love the unlovable, and to promote loving community in spite of great natural diversity.
The secondary purpose of Communion is to assimilate the body of the communicant to the body of Christ in two ways: it curbs or mitigates all disordered passions, especially those against chastity, and it confers a new title to the final resurrection of the body in heavenly glory.
A final effect of Communion is to remove the personal guilt of venial sins, and the temporal punishment due to forgiven sins, whether venial or mortal.