While baptism is a “once and for all” event that should not be repeated in the Christian’s life, the effects of baptism continue throughout the life of the believer.
Take away the communal aspect, take away the communal gathering around Christ’s body and blood, and the Christian will begin to suffer a malnutrition of faith.
This is how the God of Abraham has become our God, too. This is how God has fulfilled His promise to Abraham and His promise to us: by breaking open His own body and shedding His own blood.