You ask why we historically needed a shali'ach tzibur, when did they begin and why do we still need today?
R Ari Jacobson provides a first summary according to two different approaches
Back in the days when printed texts were not yet accessible, many Jews
didn’t own siddurim and sefarim as we do today. Although there was a
small segment of very learned scholars, there was also a significant
percentage of people who were less educated than the masses of today
and could not daven on their own. This was the original context in
which the concept of chazarat hashatz evolved. The shaliach tzibur
would repeat the text of the amidah in order to enable those who
didn't know how to pray to fulfill their obligation.
This is in consonance with the principle of shomea k’onah. If I listen
to a blessing and I have in mind to fulfill it, it’s considered as if
I actually recited the blessing myself. The source for this is in the
Navi that describes the reading of the Torah by Shafan Hasofer, the
scribe of King Yoshiyahu. The Navi says that King Yoshiyahu read from
the Torah. Actually, the king just listened with the intention of
fulfilling his obligation and it was considered as if he read the
words himself. So too, in early times, the chazzan would pray out loud
and those who couldn’t do so on their own would listen, answer amen,
and receive credit for having prayed.
Most people today pray on their own. Why do we still continue the
custom of chazarat hashatz? The Rambam has a famous responsum in which
he writes to a community that was not careful with chazarat hashatz.
He states that today there isn’t much of a reason for the chazzan to
repeat the amidah and even if we say one must hold on to the customs
of our forefathers, if people will just sit and talk it is better to
do away with the custom. Most communities did not accept this view.
There’s a second approach that suggests that perhaps the repetition of
the amidah and other prayer services is not just to help those who
cannot daven. Rav Soloveitchik notes that in the time of the Beit
Hamikdash the Jewish nation brought individual sacrifices as well as
communal offerings. Today the silent portion of the shemeonei esrei
represents the individual sacrifice and the repetition represents the
communal sacrifice. Even if one can pray on his own, there is still an
obligation of participating in the offering of the congregation.
Earlier sources such as the Beit Yosef and the Maharik reflect this
approach. For this reason, the Maharik suggests that just as in the
Beit Hamikdash, the kohen who offered the korbonot had to do so with
the agreement of the people, so too the shaliach tzibur represents the
community with his public prayers. The Avnei Nezer writes that a
chazzan should be careful not to lengthen the davening. Just as a
korbon was brought on behalf of the participants, so too the chazarat
hashatz is offered on behalf of the congregation and should be done
with their full consent.
R Adin Steinsaltz in A Guide to Jewish Prayer offers a similar view
Initially, the function of the Shaliah Tzibbur' in public prayer
services was created by reason of practical necessity. Since the
prayers had not yet been written down and many people did not know
them by heart, there was a need for someone well versed in the prayer
formulations. This person was to intone them in a strong voice, so
that others could repeat them after him, or could listen and respond
with Amen at the end of every benediction thus fulfilling their
obligation to pray. Even when the number of people familiar with the
prayers grew, every congregation still had some individuals in need of
a Shaliah Tzibbur. For this reason, it was ruled that after the
congregants conclude the silent recitation of the Amidah, the Shaliah
Tzibbur would repeat it aloud, thus enabling the general public to
fulfill its prayer obligations. This is what is called Hazarat
ha-Shatz — the repetition of the Amidah prayer by the Shaliah Tzibbur
which is an integral part of public prayer to this day (Shatz comes
from the initials of the words SHAliah TZibbur).
The repetition of the Amidah was of particular importance on festival
days, and even more so on Rosh ha-Shanah and Yom Kippur whose special
prayers were not remembered accurately by most of the worshippers, and
certainly could not be fluently recited by all (see TB Rosh haShana
34b and 35a). However, in time, as the prayers were written down
and compiled into books that were taught even to little children who
had just learned to read and prayer formulations became familiar to
all, the repetition of the Amidah continued merely as a traditional
practice, not as an actual necessity.
Indeed, Maimonides in his day ruled that this repetition should be
abolished, since everyone had already recited the Armidah in a
befitting manner; as a result, people would converse during its
repetition by the Shaliah Tzibbur, or engage in other activities,
demonstrating disrespect for the prayer and the synagogue, which mould
be considered a form of desecration. However, this ruling of
Maimonides was adhered to by only a limited number of communities,
mainly in Egypt, and perhaps also in some neighboring countries where
he was considered the indisputable authority of his day.
centuries later, the Amidah repetition was reinstated even in these
communities, as in Jewish communities everywhere else. The reason was
that, beyond being a traditional custom, the Amidah repetition had
acquired additional significance by complementing and elevating public
See also yeshiva.co where interestingly, he mentions a different Rambam that even if everyone is familiar with the prayers the regulation stays, specially since it is a congregational prayer, and the peak of the prayer is saying Kdusha with a Tzibbur.