Singing in a choir is a great enjoyment; it allows a person not only to use his or her voice, it also gives people a chance to hear and perform excellent music. Music is an important part of life and human culture, and so should be encouraged.
Choirs offer people the chance to participate in culture. Too often culture is set aside, especially in this day and age, where we equate money with society. The first thing that pops into one's mind is "I wonder how much it costs?" Culture is a thing that not only has a financial value, it also has a cultural value; that's why a car is just a car, but music is more.
Also, it allows for the voice of the local musicians and composers to be heard. For example, a church choir director needs to actively search out the music written by composers living in the area. Although performing music by great composers is perfectly fine, the contemporary voices from the area should also be heard. At least this is what I believe; it is rarely true, from my experience.
Finally, it brings music back to the home. Families used to have a piano at the house, where they would gather around the play and sing for entertainment. These days, we have mp3s and music pumped in from the outside. This is perfectly fine; I do the same thing, but it should not replace the experience of learning to read music and doing it oneself.