All of the above. It's a great way of fitting stuff into your vocal range.

I think harmonising with voice is no different to any kind of instrumental accompaniment. It can enhance the melody and enrich the sound for the listener. Whether or not any kind of accompaniment is traditional is something I'm sure we can argue about 'til the cows come home ;-)

Like any other sort of accompaniment, there's a time and a place for it - and I do think harmonisers should make sure it's OK to jump in on an unaccompanied song. Sometimes I love harmonies, but at other times, I really just want to sing stark and solo. Unless you really click musically with someone, it can be tricky to harmonise sympathetically.