Paul, just did a quick search, and found this right away. This is a topic well discussed in many trailering forums.
I really don't understand the many folks here who are thinking that the breakaway switch should take effect, only if the chains fail. I have never heard of properly attached chains failing. I have heard of, and have had myself, trailers breaking away from the hitch. The idea is to stop safely when decoupling occurs, not to stop the trailer if it is going down the road on its own.
You need the trailer breaks to lock whenever the hitch accidentally becomes detached while towing, otherwise it will come slamming back into you when it does, potentially causing a very dangerous situation. If this was to happen, that would be your first indication that it became uncoupled, and any action of yours to manually apply the brakes would come way to late, even if it crosses your thought process in an emergency situation.
If the breakaway applies the brakes should the hitch decouple, the trailer will pull straight back, allowing you to stay in full control as you come to a stop, even on rain soaked roads.
Driving professionally with vehicles equipped with breakaway controller, it was one of our walk-around checks to make sure that it in fact worked, and that it would engage should it be needed.
I have to confess, I never bothered to set up my breakaway properly on my Escape at first (knew better, but figured nothing would happen to me), and did have it decouple on the road. Fortunately it was on the service road leaving the campground, and sure enough, even at a slow speed, the trailer rammed into the tow vehicle, leaving a canoe imprint on the front window rock guard, and a dint in the Pilot bumper.