Do you hear a scritch-scritch-scritch sound coming from under your bed? Don't worry. It's just a scientists down there, recording your uninteresting, "egocentric" chatter and preserving it forever.
Ah, the 1930s were an invigorating time for science! They combined the beginnings of modern equipment and methodology with a complete lack of anything resembling ethics. For the next 40-something years, people could be drugged, diseased, and made to cut off rat heads on camera, before informed consent came in and ruined everything. While almost anyone during that time should have done almost anything to avoid a psychologist, this time the psychologists didn't give them the choice. The researchers wanted to know what people's conversations were like when they were alone. So they found a way to listen in on people when they thought they were alone.