Why do Catholics celebrate Communion at every Mass, if Christ’s death atoned for sin once and for all?
The Mass holds the place it does in Catholic worship because becomes of the Real Presence of Christ in the Catholic Eucharist. When Jesus has deigned to become present in that manner, it automatically becomes the most important form of worship.
There are many forms of Catholic worship beside the Mass, and the celebration of the Eucharist is not the only part of the Mass. But it remains the case—and rightly so—that the Mass and the celebration of the Eucharist within the Mass are, as Vatican II put it, “the source and summit of the Christian life” (Lumen Gentium 11).