Establishing a uniform formula that is repeated thrice daily in prayer has a certain disadvantage. Prayer is likely to become routine and one may lose the kavana that is aroused when one prays to God in her own words. On the other hand, had the Sages not established a fixed formula, though the righteous would utter beautiful and sincere prayers from the depths of their hearts, most people would recite hurried and brief and defective prayers.
Rambam explains (MT, Laws of Prayer 1:4) that especially after the destruction of the First Temple and the exile of Israel among the nations, this problem has intensified. Many Jews lost their proficiency in holy tongue (the Hebrew of the siddur), in which it is proper to pray. Moreover, they did not have any appropriate formulas for prayer in other languages. Therefore, the Men of the Great Assembly established the wording for all the berakhot and prayers so that all Israel would be fluent in them and so that the theme of each berakha would be stated articulately even by those who are otherwise unable to do so.
Another advantage to a fixed wording of prayer is that it includes all the general and specific needs for which it is proper to pray. Without a standard formula, everyone would presumably pray for one specific thing. Doctors would pray for the health of their patients, farmers would pray for rain, and with time, every Jew would pray only for the things close to her heart while remaining disconnected from the table of collective aspirations. The Sages therefore instituted eighteen berakhot, which incorporate all of the Jewish people’s material and spiritual aspiration. Thus, three times a day, everyone who prays balances and unifies her personal ambitions with the general needs of the nation.
In addition to whatever we understand of the fixed prayers, there are innumerable profound allusions, some of which are explained within the Jewish mystical tradition. As R. Ĥayim of Volozhin writes (Nefesh Ha-Ĥayim 2:10):
The enlightened will understand on their own why 120 sages, among them prophets, were needed to formulate a small plea or short prayer. Through Divine inspiration and supreme prophecy they grasped the order of creation and the deep mysteries of divine chariot (“the merkava”). This is why they enacted and formulated the berakhot and the prayers using these specific words – for they observed and apprehended how the light of each individual word refracts, and how each word is quite necessary to properly rectify the multiplicity of supernal worlds and powers and to bring harmony to the merkava.
He further writes (ibid. 2:13) that all the profound meanings revealed by Arizal and other saintly figures are only a drop in the sea compared to the profound innermost meaning of the Men of the Great Assembly, who instituted the prayers. Through the prophecy and divine inspiration that manifested itself upon them when they formulated the prayers and berakhot, they successfully encapsulated the rectification (tikun) of all worlds in such a way that every day new tikunim are drawn forth each day.