The article concludes:
This is the last of the eight Gregorian modes, which encapsulate and frame the potentially rigid musical structures of the entire monodic liturgical repertoire.More here.
This last mode, in the minds of the anonymous composers and of the medieval theoreticians, is a symbol of perfection, of completion, of the definitive time. The eighth mode is often an explicit allusion to the eighth day, the beginning of the new creation. It is no coincidence that the canticles and triple Alleluia of the Easter Vigil have this same modal coloring.
At the beginning of the liturgical year, Gregorian chant glimpses the outlines of the entire mystery of Christ and expands the understanding of the season of Advent to the much broader commemoration of the "Adventus Domini," an itinerary illuminated by the Paschal event that meditates upon the mystery of the birth of Jesus as much as it does upon the anticipation of his final coming.
The modal construction of this first introit is a sign of such a journey and immediately glimpses its infinite resonances.