Test all your speakers/ horns "full range" and even amps/desks/cables/etc, this little thing even helps setting up sound at gigs.
Mine had problems when it arrived, didn't work at all, the screws holding the belt clip were too long, pushing against the circuit board inside, earthing the signal, I just guessed this and removed the screws, hey presto!
I found a faulty horn in one of my PA speakers simply by plugging in and listening, BUT DO BE CAREFUL WITH YOUR EARS, one watt is enough to blow your ears as you listen closely or even not so closely, please be careful.
There's far more expensive ones but why pay more.