Short answer: To use one of the legitimate options given by the Church in order to “lighten” the liturgy during the summer weeks, especially since we sing a rather large setting of the Gloria, which is really the Sunday “hymn of praise” during the Introductory Rites of the Mass.

Long answer: The Entrance Song at the beginning of Mass serves several functions. It promotes unity by helping us to unite hearts and minds (and voices) together, it might announce the theme or focus of the liturgical day, and it accompanies the entrance procession of the ministers.
The Entrance Song may be sung by the whole congregation, by the choir alone, or by the congregation or choir in alternation with the cantor. The Church gives us four options for this song:

(1) the “proper” (specific to the day) Latin “Introit” chant from the part of the Missal called the Gradual. This would normally be sung by the choir, and we do this at St. Michael on the most solemn occasions of the year such as the Christmas Midnight Mass.

(2) a simple common (repeating for several weeks) Latin antiphon and psalm verses from the Simple Gradual, an innovation of Vatican II that is only now being explored in some places. Since it has never been translated officially, we use an unofficial version in English called By Flowing Waters.

(3) an antiphon and psalm verses from another approved collection of psalms. It is under this provision that we actually use the antiphons and psalms from By Flowing Waters.

(4) another appropriate hymn or song approved by ecclesiastical authority.

Until recently, most parishes in the United States have made almost exclusive use of the fourth option—probably because it is the recited “Low Mass” tradition that prevailed immediately after Vatican II, using the proverbial “four hymn sandwich”. The vision of Vatican II, however, was a fully participatory Missa Cantata—sung Mass—especially on Sundays. When more of the Mass itself is chanted by the priest and people, the first three options begin to feel more appropriate.

Here at St. Michael our practice is a bit of a “hybrid,” mixing and matching of options depending on the season of the year and other things going on in and around the liturgy of the day. Whatever form the Entrance Song takes, each option has a particular way of forming us spiritually as members of the Body of Christ. I’ll follow up on this question next week by considering “Responsorial Psalms in the Mass.”