New hymns can be introduced by playing a choral prelude with the melody of the hymn 4 weeks prior to the first singing of the hymn. By playing the hymn, you are putting the melody in the ears of the congregation, making it much easier for them to sing it the first time in church.
Chorale preludes are also used to introduce really well-known hymns - though it is the common practice to play just a line of a hymn in some churches, this rarely gives people time to find the hymnal, open it and find the hymn, causing them to be frustrated and not sing.
Playing just the lsat line of a hymn is even worse - playing the first line can trigger a person’s memory, causing them to think, “I know that one!”, and want to sing, but last lines only work on Holy God, We Praise Thy Name!
Though they are called preludes, they may also be used as postludes after church, with a simple substitution* of stops. You’ll be surprised at how a few stop changes will totally change the character of these preludes.