The Watchtower claims the purpose is to cause those who stray to see the error of their ways and repent but the real truth is that the purpose of shunning is to keep witnesses in fear of leaving if they start having doubts. It is a very effective way of controling people.
Recently on this board I mentioned my second cousin on my wife side of the family. When she left about four years ago her parents turned their back on her. She knew I was a Jdub at one time who left and she wanted to contact me. No one would give her my number. My wife had her number but didn't want to give it to me. They still try to control you after you leave. It really is quite the evil system.
One channel on Direct TV is running a program called " The Mormon Candidate". It is about Mitt but it cover how Morons treat those who leave the church. Very similar to what the Dubs do.