Here’s where we get “source and summit” in Vatican II, one of the most-quoted sections of the whole constitution:
Nevertheless the liturgy is the summit toward which the activity of the Church is directed; at the same time it is the font from which all her power flows. For the aim and object of apostolic works is that all who are made (daughters and) sons of God by faith and baptism should come together to praise God in the midst of His Church, to take part in the sacrifice, and to eat the Lord’s supper.
Sacrifice and meal, right?
The liturgy in its turn moves the faithful, filled with “the paschal sacraments,” to be “one in holiness”(Postcommunion for both Masses of Easter Sunday.); it prays that “they may hold fast in their lives to what they have grasped by their faith” (Collect of the Mass for Tuesday of Easter Week.); the renewal in the eucharist of the covenant between the Lord and (humankind) draws the faithful into the compelling love of Christ and sets them on fire. From the liturgy, therefore, and especially from the eucharist, as from a font, grace is poured forth upon us; and the sanctification of (people) in Christ and the glorification of God, to which all other activities of the Church are directed as toward their end, is achieved in the most efficacious possible way.
This seems fairly straight-forward: the two-fold purpose of liturgy, elucidated here: the sanctification of the believers, the glorification of God.