They help, a little bit, but the primary way they help is it makes you feel like you are doing something. Like maybe you are bugging the dog a bit in payback for the unceasing background barking.
I have a device called a Barx Buddy and agree that one benefit is that it makes me feel like I have a little control in the situation. Kind of like how I like to corral cell phone talkers in stores--as they're mindlessly strolling around yammering and bugging me, I approach them from various sides as if I'm just walking around, too, and make them move in a certain direction without their even knowing it.
I live in an RV and am subjected to a constantly changing array of barking dogs, usually in pretty close proximity.
With every dog I've tried the Barx Buddy on, I could tell from their reaction that they heard something. Sometimes they would quit barking entirely, and other times they would react, pause, and then start barking again--that happened especially with a little yapper that was so agitated it was destroying the blinds on the window it was looking out. When the owner came back, I told her I felt sorry for her dog because it was obviously suffering to be jumping and running and barking so frantically, but she claimed the dog didn't bark when she's gone.
But the biggest success ever with the Barx Buddy was recently, when I was staying in my motorhome at a repair shop. They had two dogs behind a chain link fence that would suddenly go bananas with howling and barking for no apparent reason. They were about 15 feet from my RV, and I hit the button while pointing it through the windshield and they stopped INSTANTLY, and just turned around and went back to their houses. It was thrilling.
Other times I'd open the window and direct it in their general direction and they stopped INSTANTLY. I kind of wonder if they reacted so "well" because they'd been trained to be guard dogs, where they're supposed to bark until told not to, and they knew the sound meant stop. It was pretty clear the little yapper who ignored the sound hadn't been trained to stop barking.
I'm looking with interest at the Stunick device linked to above, because I'm sure the Barx Buddy's range is more limited than that. But I'm telling you, even if the damn dog doesn't stop barking, I can't begin to describe how satisfying it is to know that you're getting into the beast's mind even a little bit.