I believe that the reed switch module is adapted from a common module designed to read sensors with an analog output(like a photosensor or ldr) which is then fed to a comparator(lm393) for conversion to a digital output(DO) at a level set by the potentiometer. It's evidently inexpensive enough to use it as a mounting platform for something which does not require the circuitry.
If you are not seeing HIGH/LOW output on DO with the magnet near and far, you might have to adjust the potentiometer to get the digital output when the switch is open or closed.