Free showdown

I've seen a few posts about raising the turn on a semi-bluff. I assume you wouldn't do this on a draw, but rather a weaker hand that what you were representing. Then checking the river you would have called, so its not really a free showdown, but if you were going to call both streets it doesn't lose money. Are there scenarios where this would clearly be the right play? I just can't see where I might use this and was looking for some input.