Initially, the chorales were created from popular secular songs. These chorales became very popular in the church because the congregation could relate the chorale to the tune that they already knew. Bach then saw that he could include a tune from a chorale into a passion for the same reason, so that his audience could relate back to church. Passions tended to be long works and to keep the audience interested in the piece, Bach could add a known chorale and keep the audience entertained with music that was familiar to them. For example, Bach in the second section of the Passion uses the popular hymn "Christus der uns selig macht" (Christ, who makes us blessed, No. 21). Although a familiar hymn to the audience, the chorale is arranged into a motet setting to create further interest for the audience.
Furthermore, the use of a chorale in a passion would also serve to remind the audience of the religious origins of the music. A chorale heard in a passion would remind the listener that it was originally heard in church, thereby reinforcing the message of the original text from the church hymn but also the message of the new text in the passion.
In modern day performance, the congregational chorales of the St. John Passion are unaccompanied. However, Bach did intend for these chorales to be orchestrally accompanied in concert. For example, the congregational chorale "Wer hat dich so geschlagen" (Who has struck Thee so, No. 15) was clearly intended to be accompanied by the full orchestra. Bach wrote parts for the orchestral instruments and the parts are marked accordingly as to where to start playing to accompany the choir. Furthermore, the audience was expected to join the choir and sing along. The choir was simply used to lead the congregation in the performance of this chorale in the passion.
Chorales were also seen as a way for Bach to add in extra material that he did not compose himself. Original composition was a time-consuming process, and he could ease the burden on himself by adding chorales. From time to time, Bach would actually exchange the chorales in the passions if he felt a certain chorale would suite a performance better. For example, the chorale chorus "O Mensch bewein dein Sünde Groß" (O man, thy heavy sin lament, No. 35 of the St. Matthew Passion), was originally the introduction of the St. John Passion, but is now the conclusion of the first part of the St. Matthew Passion.