Fwiw, here's something I posted elsewhere
on the forum...
The Orthodox Church uses fixed prayers for a variety of reasons. First, fixed prayers such as the Psalms have long been a part of Christian prayer life, dating back to the New Testament times (Eph. 5:19; Col. 3:16; James 5:13), and we also see biblical evidence for continuing the practice of praying at appointed hours (Acts 3:1; 10:9, 30). Second, Jesus Himself, in responding to the Apostles request to be taught how to pray (Lk. 11:1), gave a fixed prayer, saying “This, then, is how you should pray,” and then proceeding to give us the Our Father (Matt. 6:9-13; Lk. 11:2-4).
The reason that this is beneficial is that it cuts out some potential for self-willed prayer. Many Orthodox prayers come down to usfrom holy men, and the prayers have been tested and sanctified by use throughout the centuries. What we have in prayer books are not a random collection of prayers randomly selected. No, the Church, through the guidance of the Holy Spirit, has selected specific prayers because they help guide us towards truths about God and His creation. This is not to say that there are not sometimes issues, only that the Church in it’s collective wisdom is a better discerner of how to best pray to God than we are as individuals. Thus using fixed prayers is also an exercise in foundational Christian virtues such as humility and obedience.
Fixed prayers also connects people of various cultures, nationalities, geographic locations, and languages. True, any prayers could unite the body of Christ, so long as they were sincere. But using prayers known across boundaries of culture, language, etc. has a practical as well as spiritual benefit. So to with the liturgies, while there are some variations, and changes occur over the centuries, yet there is also a great deal of consistency from one year to the next. The Apostles didn’t switch worship styles based on popular opinion, and neither do the Orthodox. Thus the Orthodox continued the style of worship that they had received, ie. liturgical worship (Acts. 13:2), they established customs to be kept (1 Cor. 10:16-17; 11:2, 20-30), and as St. Paul put it: "Let all things be done decently and in order." (1 Cor. 14:40)
Of course, one problem that Protestants often have when they first explore Orthodoxy prayer and worship is the use of repetition. After all, did not Jesus say: “But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking.” (Matt. 6:7; cf Eccl. 5:2)? They key word here is “vain,” however. Not all repetitions are vain, as we can clearly see from other Scriptural passages. In the Psalms we often see repetitions, sometimes of subject material (e.g. Ps. 119), and sometimes through using a refrain, such as when Ps. 136 finished every verse with: “for his mercy endureth for ever”. We should also remember that St. Paul spoke of the idea that we could “pray without ceasing” (1 Thes. 5:17), which clearly would require some repetition, no matter how creative you were. And perhaps most importantly, Jesus Himself repeated his prayers on the night of His betrayal (Matt 26:37-45; Mk 14:33-42).