Anshei Knesset HaGedolah established that after individuals finish reciting the silent Shemoneh Esrei, the chazan repeats the Amidah out loud in order to fulfill the obligation of prayer for those who do not know how to pray by themselves (Rosh HaShanah 34b). This repetition is known as Chazarat HaShatz. However, for Ma’ariv, they did not institute an Amidah repetition since, in essence, Ma’ariv is an optional prayer (although nowadays it is an obligatory prayer, as explained further in this book 25:2) and consequently, there is no need to fulfill the obligation for those who are not well-versed in the passages of the prayer service.
The Chachamim instituted that the chazan also prays the silent Amidah in order to organize the prayer in his mind. Additionally, they instructed that even those who know how to pray by themselves listen to Chazarat HaShatz and answer Amen after the berachot.
Because the Amidah repetition was enacted by Chazal, it must be recited even in a minyan in which all the people know how to pray by themselves. Even after it became permissible to put Torah Sheb’al Peh (the Oral Torah) into writing, including the wording of the prayers, and though the use of siddurim has become common, the enactment of the Chachamim did not change. Moreover, today, when one rarely finds a minyan where someone needs to fulfill his obligation with Chazarat HaShatz, it is nevertheless recited, for the rule is that once the Chachamim enact a law, distinctions are not made between cases in which the law applies, and those in which it does not (Shulchan Aruch 124:3, based on the responsa of the Rambam). Further, the Chachamim instituted reciting Kedushah and Birkat Kohanim in the repetition, and if the Amidah is not repeated, they will not be recited altogether (Tur).
The Kabbalah clarifies that in addition to the simple explanation of why both the Amidah and its repetition are necessary, namely, to fulfill the obligation of one who is not proficient in the wording of the prayers, there is a sublime reason according to the secrets of Torah (sod), that the recital of both prayers together causes them to be more effective. Therefore, even today, when there is no need to fulfill the obligation of one who is not well-versed in prayer, the chazan must still recite the Chazarat HaShatz, for the hidden reason still stands.
It is a great privilege to answer Amen to the berachot of the Amidah repetition. Even learning Torah is forbidden at that time (see Kaf HaChaim 124:2 and 16). The virtue of the Amidah repetition is greater than the virtue of the silent Amidah. Therefore, although the themes of the Amidah are supremely recondite, permission is granted to recite it aloud. Due to its profound virtue, there is no concern that forces of impurity will take hold of it. The listeners must refrain from invalidating it by talking. It is said of one who chatters during Chazarat HaShatz “that he is sinning, and his transgression is too great to bear; therefore he must be rebuked” (Shulchan Aruch 124:7).