Brian Gaff formulated the question :
why not just use the door to control a switch like they do for the fridge
light, never did understand why a few pence on such a device was not on all
freezers.

It already has a built in 'temperature too high' warning light, but
when lit it is not obvious unless you look at it with the door open and
when you are in the utility room. I wanted an audible means of knowing
when in the kitchen, that the door was left open or the temperature in
the freezer went to high. So this serves to warn if the door has been
left open, power has been lost, or the freezer has suffered some other
failure.