I suppose Musicam Sacram is as good a starting place as any. Paragraph 19 says that the role of the choir is "to ensure the proper performance of the parts which belong to it, according to the different kinds of music sung, and to encourage the active participation of the faithful in the singing." In recent decades, the secondary role has often overshadowed the primary role. Many music directors insist on amplified cantors and/or choral harmonization all the time and seem to forget that the congregation has its own voice. Pardon my informal terminology, but choirs really need to focus primarily on the non-sing-along music.

Pope John Paul II wrote/said a fair amount about this. He discussed how the choir can be used to foster "full and active participation" by promoting quiet and reflection, which is something actively discouraged by our culture.

There are small churches, and large churches with many masses but not the resources to treat them all with equal solemnity. (eg Westminster Cathedral)

GIRM 104. It is fitting that there be a cantor or a choir director to direct and support the people’s singing. Indeed, when there is no choir, it is up to the cantor to direct the different chants, with the people taking the part proper to them. 88
87 Sacred Congregation of Rites, Instruction, Musicam sacram, 5 March 1967, no. 19: Acta Apostolicae Sedis 59 (1967), p. 306.
88 Cf. ibidem, no. 21: pp. 306-307

Some years ago I wrote a short piece for CCWatershed on Musicam Sacram and the role of the choir in the Mass. I hope it may be helpful.