What is the purpose of the buzzer?

Answer 1

an electro- mechanical buzzer is a cheap way to generate a sound from a DC source. They were also used in early automotive electrical systems to create an AC voltage that the coil could increase to ignition system levels.
Another use was to again provide AC correct for the primary of power supplies for tube type radios in cars.

Answer 2

It is a leftover from the days when there were no better ways of seeing who was at the backdoor. If they had closed circuit TV then there would never have been that annoying buzz sound.

Answer 3

To stimulate your little tushie.