R. C. Sproul has some instructive comments on why we say "Amen" in our corporate worship. He begins,
And all the people said… “Amen!” The “amen corner” has had an important
place in the life of the church throughout the ages. However, it is rare
to find such a spot among Presbyterians. We are known as God’s frozen
chosen for a reason. It has been said that the Methodists like to shout
“Fire,” the Baptists like to shout “Water,” and the Presbyterians like
to softly say, “Order, order.” Nevertheless, in spite of the
idiosyncrasies of various ecclesiastical persuasions, the function of
the word amen far transcends denominational usages in the modern era.
Read the whole thing.