From Jason McFarland’s book:
Without a doubt, responsorial or antiphonal singing is especially suitable for processions because such songs have an open musical form and thus a variable length that can accommodate the infinitely variable length of an entrance procession. Repeated antiphons also facilitate active participation because the gathered faithful can sing them from memory, in contrast to singing a hymn from a book, and thus engage more fully in the entrance procession. (p. 110)
Indeed, antiphons are a unique liturgical-textual genre. Their purpose is neither to proclaim nor to allude to the ancient Christian textual tradition, but to appropriate it. In so doing, antiphons reflect and pass on a tradition of textual interpretation. This occurs through the way a specific antiphon text employs the textual tradition and relates to and interacts with its verses to produce a particular Christian meaning, and then how the antiphon and verses together relate to and reveal something about a specific day of the liturgical year, thereby announcing “the mystery of the liturgical time or festivity” (GIRM 47). (p. 167)