It seems to me that there are four main areas of benefit to singing in a group:

  1. social — singing is a community activity
  2. personal — singing creates a sense of achievement
  3. musical — singing together makes for a great sound
  4. well-being — singing is good for your health

1. Singing is a community activity

   * you will make friends who have a shared interest
   * working as a team — every contribution counts, but the team must pull together as one
   * creating a community of like-minded people with a shared goal
   * you will meet people from a range of different backgrounds
   * and for you men out there: there are always far more women than men in mixed choirs!

2. Singing creates a sense of achievement

   * when you finally nail a difficult song
   * constantly improving your singing technique
   * regularly developing your vocal skills
   * learning new skills and languages
   * remembering words and melodies
   * being challenged by more difficult material
   * performing in a concert in front of an audience

3. Singing together makes for a great sound

   * being part of a big musical sound
   * the pleasures of harmonising with others
   * creating beautiful music together
   * rehearsing to make a song sound really good
   * getting positive feedback from audiences after working hard on a song

4. Singing is good for your health

   * warm-ups improve the body’s flexibility, sense of rhythm, balance, self-awareness, etc.
   * breathing techniques help to reduce stress and relax the body
   * expressing yourself through sound helps to release emotional blockages
   * being part of a group working together creates a sense of belonging, community and shared             endeavour
   * singing is an aerobic activity that increases oxygenation in the blood stream and exercises               major muscle groups in the upper body
   * singing has a positive effect in reducing stress levels through the action of the endocrine                    system which is linked to our sense of emotional well-being
   * singing can increase lung capacity, improve posture, clear respiratory tubes and sinuses, and          can increase mental alertness through greater oxygenation
   * singing promotes both a healthy heart and an enhanced mental state as well as improving your         immune system — so you will live longer!
   * recent Swedish research has shown that choral singing has strong effects on-being, in that                positive emotions increase significantly, and, in turn, negative emotions radically subside.                 Choral singers, particularly women, are happier, more alert and relaxed after a rehearsal.

Here is the citation - 
http://singing-thoughts.blogs well pot.com/2008/10/there-are-plenty-of-good-reasons-to.html