Since Abraham, intercession - asking on behalf of another has been characteristic of a heart attuned to God's mercy. In the age of the Church, Christian intercession participates in Christ's, as an expression of the communion of saints. In intercession, he who prays looks "not only to his own interests, but also to the interests of others," even to the point of praying for those who do him harm.
Source: Catechism of the Catholic Church (Vatican.va)