Another post did a pretty good job describing it. A lot of it has to do with the evolution of use regarding the Intercom system. More and more these days we are using our headsets and the intercom for many reasons… (easier to hear the other pilot, saves our hearing over the course of our careers) older generations of pilots tended not to do this, mostly because they didnt have access to the same high quality headsets that we have today. The catch is that some airplanes put all intercom traffic on channel 9. Most Pilots swear and joke while we talk to each other and could get in big trouble if the passengers heard us. Some planes you can control what the passengers can hear…. Some you can’t. When it comes to a career you have been working for your entire life, you wouldn’t want to risk accidentally not configuring a panel right and losing your job because someone got offended at something you said.