Explain the use of the chazzan to end paragraphs

Question

My understanding of the "history" of the shaliach tzibbur (shat"z) is that few people could read or pronounce Hebrew, and, there were few siddurim printed. Thus, the use of an "expert" to represent the congregation in prayers. I gather that the primary usage of the chazzan was for the amidah as we still have the repetition of the amidah, now, even though we also have an individual, silent one.

I assume that other parts of the prayer service such as Psukei Dezimra / the blessing before and after shema, etc. were all done by the chazzan, originally, and aloud, assuming that few spoke Hebrew (I'm uncertain about the verses of Shema, itself, as I don't think the shat"z can fulfill another's obligation to recite Shema.)

Now, that much of the prayers are recited silently, Since when has the chazzan been used as a means of reciting the end of each Psalm, paragraph? What is the purpose for this? Why can't, say, everyone recite silently Psukei Dezimra or the Shema blessings silently and the chazzan should do aloud only Amidah and Kaddish?

Notes:

• I have seen in many Hassidic shuls (shtiebles) that the shat"z doesn't go to the amud until Yishtabach, so, in fact, Psukei Dezimra is recited "silently". This isn't a universal method, though. Many shuls havethe shat"z recite the end of each paragraph
• I assume that the reason fro having the shat"z recite the Shema blessings aloud is so that the congregation can answer Amen. Is that the only reason?

Please correct me and / or edit any incorrect assumptions that I've made.

Answer 1

A Chazzan's job is to recite the Shema Blessings and Amida sections aloud for the congregation to fulfill their obligation with. (For the Amida, those who know how to pray themselves must also do so individually seemingly since prayer is inherently personal.) Congregations who choose to utilize a "Chazzan" during Pesukei Dizimra are essentially just using a glorified human page number sign perhaps to give people a sense of community or recommended pace. The real Chazzan starts before Shema, which is why there are traditional grandiose entrances for him on special occasions (Shokhen Ad, HaMelekh, etc.) as well as opening with the formal call to prayer "Barekhu".

As noted elsewhere if one fears spacing out during the blessings surrounding Shema (or during a Zimmun) and not hearing every word, he should recite along with the leader in an undertone, but finish up a few seconds early so as to still say Amen.

So to answer your question: Why can't, say, everyone recite silently Psukei Dezimra or the Shema blessings silently and the chazzan should do aloud only Amidah and Kaddish?

For Pesukei Dizimra you certainly can do that if you want, and some synagogues indeed do. For the Shema blessings you can't do that since the Chazzan needs to say them aloud for the congregation just like he does the Amida. Even if it happens that everyone in a particular synagogue fears spacing out and recites along themselves, we don't change the order of the prayers (just like we don't abolish the repetition of the Amida if everyone in the room knows how to say it themselves) since that is the traditional law, communal prayer has a special status, we'd need to say the Barekhu/Kedusha parts of it anyway, and people might show up one day who can't or merely don't want to say it themselves.