Alevis call the saz the stringed Quran, and the phrase is not a flourish. Alevi religious teaching has been carried in sung Turkish poetry rather than in Arabic legal texts, and the instrument on which that poetry is performed is treated with a reverence that Sunni jurisprudence reserves for the book. Critics of the tradition have taken the expression as evidence of heterodoxy, which tells you how much weight it carries on both sides.
The performance belongs to the cem, the communal ritual, and it belongs to it structurally. The cem is organised around a set of offices, and one of them, the zakir, is the musician. He is not accompanying the service. Singing the nefes is one of its constituent duties, in the same way that the presence of the dede who leads it is a duty, and a cem without it is incomplete rather than plain.
The content of the repertory is doctrinal, devotional and often political at once. Its poems address the household of the Prophet and the twelve imams, they teach the ethical demands the tradition makes on its members, and a substantial part of the canon is the work of poets who were executed. Pir Sultan Abdal, the best known of them, was hanged in the sixteenth century after siding against the Ottoman state, and his poems have been sung as protest for four hundred years since, including by Turkish left wing movements in the twentieth century that held none of the theology. That double life, as liturgy and as protest song, is one of the things that makes the nefes hard to file.