I don't know what they use them for, but I do know that just about every player on the Nationwide Tour was carrying a weighted swing trainer in his bag when they visited Boise for the Albertson's Open. I got to see this because I was there for the practice rounds. You will never see one on in the bag on TV because it would count against the limit of 14.

Anyhow, there must be something about them that really good players like. My guess is that it is more about warm-up than swing plane... but only a sub-par golfer could really answer the question.