Headphones originated from the telephone earpiece, and were the only way to listen to audio signals before amplifiers were developed. The first truly successful set was developed by Nathaniel Baldwin, who made them by hand in his kitchen & sold them to the U.S. Navy.
In early powered radios, the headphone was part of the vacuum tube's plate circuit & had dangerous voltages on it. It was normally connected directly to the positive high voltage battery terminal & the other battery terminal was securely grounded. The use of bare electrical connections meant that users could be SHOCKED if they touched the bare headphone connections while adjusting an uncomfortable headset.