Teaching and admonishing one another in psalms and hymns and spiritual songs.
Chant carries the rite
The Coptic rite is full of exchange: priest, deacons, and people. Chant gives that exchange order. It helps the deacons lead the responses, helps the people answer, and lets Scripture, litanies, psalms, Gospel prayers, and Eucharistic prayer keep their sacred character.
Many parts of the Liturgy return week after week. The melody becomes a way of remembering doctrine, Scripture, and the order of worship. The faithful do not only read the faith; they hear it, repeat it, and carry it in memory.
Different chants teach different moments
Chant is not one sound used everywhere. A Gospel response, a procession, a Lenten hymn, a Paschal hymn, a Communion hymn, and a litany each teach a different kind of attention. The melody helps the faithful know whether the Church is praising, repenting, proclaiming, interceding, or preparing to receive Christ.
This is why the Coptic Church preserves hymnody carefully. The melodies are part of the received way the Church prays the faith, not a layer added after the teaching is finished.