Teaching a trust fall (and rewarding it heavily) is a great way of improving your dog's confidence and faith in you, as it shows them that if they do something that appears scary, i.e. falling into the unknown, bad things don't always happen.
The idea of falling backwards is scary for most dogs, but actually falling backwards onto a soft surface and then subsequently getting a big fuss and some treats really isn't that bad!
I have seen dogs that have been taught this trick are more likely to be confident when learning new things, as they have overcome the "what if?" barrier. Even if the concept behind what is being asked may usually hold them back, dogs will learn that you won't ever ask them to do something that is too much.
What we absolutely have to do when training this is
1) Do it on a soft surface where the dog has no chance of hurting themselves
2) Reward heavily and keep it fun
3) Don't expect them to fall backwards immediately, reward them for shifting their weight back onto their hips to start off with and build it up
4) If they find it too much, stop. This should be a game, not something stressful.