In this question , the accepted answer explained that we have a shaliach tzibur in minyanim today, even if everyone knows how to pray, for two reasons:
Minhag
We don't want to embarrass someone by asking if anyone needs a shaliach tzibur, so we always have one, and if someone doesn't know how to pray, he can fulfill his mitzvah of tefilah through the shaliach tzibur.
Can someone fulfill the mitzvah of tefilah by listening to the shaliach tzibur and answering "amen", even if he knows how to pray?