http://www.nbcnews.com/news/us-news/bomb-dogs-failed-dozens-tests-10-large-u-s-airports-n648466 So, they do nothing, much as the screening does nothing aside from forcing a bunch of people to share the smell of their feet. Surprise, surprise. Of course, it's Obama's fault.
For what its worth I was interviewed and then tested as a TSA screener some years ago. So it could be worse.