Choke chains are a useful way to teach your dog that straining harder is not the right way to get what he wants. Instrumental as it is in training, remember that they can get 'caught' easily.
What this means is, that after being pulled taught, the chinks in the chain can become entangled so as to prevent the chain from loosening again. This is even more likely to happen with rusted chains. So make sure that you don't leave your dog chained up and unattended with a choke chain around their neck. They may actually get chocked.