From time to time, I'm told that the Graduale has been depreciated in favor of the Responsorial Psalm, and this is always said as if to suggest that the Graduale, while permissible, must be looked down upon as an inferior option - even though these are among the oldest in the Gregorian books and even though they are unquestionably the great masterpieces of all the books.
Well, here we go: the Graduale is being sung again in the Vatican. Here is Christus Factus Est for Good Friday (commentator below points out that this is actually the Graduale for Palm Sunday and it is being sung to the text of the Gospel acclamation). The "function," if we can call it that, of the Graduale is to inspire reflection on holy scripture. As William Mahrt points out, it is clearly the case that the music is elevated even above the text as the preeminent thing in these pieces, as illustrated by the luxuriating melismas that occur throughout the pieces. These pieces clearly depart from the formula of the melody being a vessel for the more-important text; the music here becomes the "text" which is to say that the music here is the message, the purpose, the driving functional element, and the long elaborations on single syllables are structured create an earthly stillness so that the mind and heart can be elevated to the heavens to prepare us.