I am involved in Sacred Harp singing which is a tradition from the Southern USA (and New England) but which has now become popular in several other countries including the UK - where we have a small but very enthusiastic group of singers! Sacred Harp (referring to the human voice but also the hymn book of the same name from 1844)is four-part unaccompanied harmony and personally I find it a very powerful and spine-tingling musical form. It is very much a participation rather than a performance thing and it is all about belting it out in full-throated voice rather than the more controlled style of a choir. To my shame I don't know that much about the reasons for singing unaccompanied but I believe that Sacred Harp traces its origins to Britain and possibly especially to the Western Isles tradition that has been mentioned above.

If the OP wants more info on Sacred Harp, check out this site:

http://fasola.org/