Citizens tell dirty jokes to remember the good times.
At a time of deperation, he whispers this prized piece of information to suggest there are better times coming, and before you know it, everyone knows about Jakob's radio. Of course, Jakob doesn't really have a radio, but that's not what these desperate folks want to hear. Although he tells them over and over, nobody listens. The Nazis didn't exactly hand out radios. In fact, owning one in the Ghetto was a capitol crime.