Why Are Mass Intention Offered?
When a priest offers Holy Mass, he has three intentions: First, to offer the Mass reverently and validly in accord with the norms of the Church. Second, to offer the Mass in union with the whole Church and for the good of the whole Church. Third, to offer the Mass for a particular intention, such as the repose of the soul of someone who has died, in thanksgiving for recovery or some special favor, petition for some favor, birthdays, anniversaries, etc.
Therefore, the effects of the Mass bring certain benefits or fruits. The general fruits of the Mass are the effects upon the whole Church — to the living faithful as well as the poor souls in purgatory. For this reason, in the Canon of the Mass (the Eucharistic Prayer), a special mention is made for both the living and the dead.
The special ministerial fruits of the Mass are applied to the particular intention of the Mass, i.e., “for whom the Mass is offered.”