From the wise and wonderful blogger of Hymnography Unbound, this gem, in a combox at the CMAA discussion boards:
Two things can be "caught"--as distinct from "taught"-- by the propers
a) correct relationships, both human-divine and ecclesial, and
b) the elevation of desires.
In other words, the propers form the mind for life in the reign of God.
And that's just the words!
The singing, and the ecclesial presence brought about by singing, form the whole human being:
body, soul, and spirit.
You just might be formed differently by giving voice to the sentiments, "Let the hearts of those who seek the Lord rejoice; seek the Lord and be strengthened," than you would by, say, joining in the national anthem of the nation of Pelagia, which begins, "Let us build a house....," or celebrating the wonderfulness of yourselves, "We are called, we are chosen, we are..."