Something I wrote on the topic a few years ago:
Tefillat hatzibbur b’tzibbur – There are different notions regarding the essence of davening and the role each of us plays in creating an atmosphere of kedusha. According to the rav’s interpretation of the rambam (hilchot tefilla in general, and the presence of perek 9 which details how the davening of the shali’ach tzibbur is to progress, right after the purpose of this davening is supposedly covered in 8:9) there are three different types of davening – that of the yachid, which stems from the loshon of vaye’etar Yitzchak in bereishit 25:21, that of the tefilla of the yachid b’tzibbur, which has at its heart the members of the minyans’ reciting the shmoneh esrei silently but together [as per the mishna berurah’s statement (siman 90, se’if 9, se’if katan 28 citing the chayeii adam) which clarifies that the point of a minyan is not the answering of omein and the permission to recite devarim sheb’kedusha, but the power of the collective saying of the amidah which makes b’rov am – as the text says in se’if 9: a man should try “lehitpalel…im hatzibbur” and this is also clear from M”B 66:35 “gam lechatchilah ra’uii l’hatchil…im...”] and the third, a separate concept of the repetition of davening by the shali’ach tzibbur in the presence of the minyan.
The role of the shali’ach tzibbur’s davening is, therefore, two fold, and this double use creates halachic ramifications. According to the rambam in 8:9 (and the special exceptions in 8:10) the davening of the shali’ach tzibbur is to exempt those people in the kahal who are present and who answer omein (as per hilchot berachot, 1:11, answering omein is equivalent to having said the bracha) and who are not knowledgeable enough to say the davening on their own (gemara rosh hashana 34b). Were this the only purpose of the repetition for the amidah, then in those situations where the kahal has no one ignorant in it, there would be no obligation for chazarat hashatz. In fact, this understanding has driven the minhag in yeshivot to recite only the heicha kedusha (tefilla ketzarah) even when there is no sha’at had’chak or fear of not having a minyan, as their assumption is that the role of the shatz in being motzi someone is not necessary (from a drasha citing rav ya’akov komeinetzky, however another drasha I read cites the Shut Az Nidbiru (XII:23) from Binyamin Yehoshua Zilber as criticizing yeshivas for this practice and assumption). Additionally, if the individual were only there as a member of a minyan to allow the shatz to recite the chazorah then the individual’s level of attention and involvement would need to be cursory at best. The text, though seems to demand more of the person, making it seem that his place within this avodah frame is not as a number, but as an involved party.
The rambam’s writing of perek 9, therefore, must lead to new insight – especially halacha two which supposes a participatory value for both one who is unfamiliar with davening and one who is knowledgeable and halacha three which demands that all stand and respond. Therefore the rav deduces that this davening by the shatz must have its own value – it must stand in place of a korban tzibbur (especially the bracha of “r’tzei” which acts through this tefillat hatzibur as allowing nesiyat kapayim after a korban tzibbur) which is separate from that of a yachid, even a yachid b’tzibbur; it must represent the community is a unified voice which then has the power to demand on behalf of the kahal in a way that they could not individually. This understanding of the role of the shatz places new responsibilities on the individual. Instead of being a passive observer who, at best, responds omein, the individual must become a participant, keeping his physical and mental attitude the same as if he were saying the words, as he is trying to be made yotzei a completely new obligation. Interruptions such as talking and learning invalidate that attitude as might the saying of modim derobonon so loud that it makes one not hear the modim of the shatz, as hearing every word of his davening is intrinsic to fulfilling this obligation.