The liturgy may be called upon to assist a theological argument:
48. For this reason, whenever there was question of defining a truth revealed by God, the Sovereign Pontiff and the Councils in their recourse to the “theological sources,” as they are called, have not seldom drawn many an argument from this sacred science of the liturgy. For an example in point, Our predecessor of immortal memory, Pius IX, so argued when he proclaimed the Immaculate Conception of the Virgin Mary. Similarly during the discussion of a doubtful or controversial truth, the Church and the Holy Fathers have not failed to look to the age-old and age-honored sacred rites for enlightenment. Hence the well-known and venerable maxim, “Legem credendi lex statuat supplicandi” – let the rule for prayer determine the rule of belief.[De gratia Dei “Indiculus.”] The sacred liturgy, consequently, does not decide or determine independently and of itself what is of Catholic faith. More properly, since the liturgy is also a profession of eternal truths, and subject, as such, to the supreme teaching authority of the Church, it can supply proofs and testimony, quite clearly, of no little value, towards the determination of a particular point of Christian doctrine. But if one desires to differentiate and describe the relationship between faith and the sacred liturgy in absolute and general terms, it is perfectly correct to say, “Lex credendi legem statuat supplicandi” – let the rule of belief determine the rule of prayer. The same holds true for the other theological virtues also, “In . . . fide, spe, caritate continuato desiderio semper oramus” – we pray always, with constant yearning in faith, hope and charity.[Saint Augustine, Epist. 130, ad Probam, 18]
What do you make of this statement:
49. From time immemorial the ecclesiastical hierarchy has exercised this right in matters liturgical. It has organized and regulated divine worship, enriching it constantly with new splendor and beauty, to the glory of God and the spiritual profit of Christians. What is more, it has not been slow – keeping the substance of the Mass and sacraments carefully intact – to modify what it deemed not altogether fitting, and to add what appeared more likely to increase the honor paid to Jesus Christ and the august Trinity, and to instruct and stimulate the Christian people to greater advantage.[Cf. Constitution Divini cultus, December 20, 1928]
Has the hierarchy really been “not slow” in liturgical reform?
You can consult Mediator Dei on the Vatican web site here.