The Men of the Great Assembly ordained that after individuals finish reciting the silent Shemoneh Esrei, the shali’aĥ tzibur (abbreviated to “shatz” an meaning “envoy of the community”; it refers to the ĥazan) repeats the Amida out loud in order to fulfill the obligation on behalf of those who do not know how to pray on their own (RH 34b). This repetition is known as Ĥazarat Ha-shatz. However, for Ma’ariv, they did not institute an Amida repetition since, technically, Ma’ariv is voluntary and consequently there is no need to fulfill the mitzva on behalf of those who are not well-versed in the prayer service.
The Sages ordained that the ĥazan also prays the silent Amida in order to prepare for his repetition. They also instructed that even those who know how to pray on their own listen to Ĥazarat Ha-shatz and answer “amen” after every berakha.
Because Ĥazarat Ha-shatz was ordained by the Sages, it must be recited even where the congregants know how to pray on their own. Even now that it is permissible to put the Oral Torah, including the prayers, in writing, and even now that siddurim are so ubiquitous that it is rare to find a minyan where someone needs Ĥazarat Ha-shatz to fulfill his obligation, the Sages’ enactment remains in force. The principle in play is that once the Sages enact a law, they do not distinguish between individual cases (SA 124:3, based on Responsa Rambam). Moreover, the Sages instituted Kedusha and Birkat Kohanim in Ĥazarat Ha-shatz, and if the Amida is not repeated, they will be abolished altogether (Tur).
Kabbalistic literature explains that in addition to the simple explanation – that it allows the uneducated to fulfill their obligation – there is another, esoteric reason. It is necessary for there to be a silent Amida as well as Ĥazarat Ha-shatz because through both of them together, prayer is most effective. Therefore, even where there is no need to fulfill the obligation on behalf of one who is not well-versed though Ĥazarat Ha-shatz, we must recite it, for the esoteric reason remains. Ĥazarat Ha-shatz is of even greater value than the silent Amida as it is collective and the congregation answers Amen to its berakhot. Hence, although the Amida is intense and sublime, and in order to preserve its exaltedness one recites it silently, the ĥazan recites Ĥazarat Ha-shatz aloud, for due to its even greater value no concern that the kelipot (“husks”; a kabbalistic term for the forces of evil) will take hold of it (see SA 124:7; Kaf Ha-ĥayim 124:2 and 16).