I see a lot of that in the WWII Allied propaganda posters. And I understand why ranking officials should obey this, but these posters were distributed to the general public. So my question is why? What exactly did the average person know that would help the Axis powers?
The main reason behind why the average person was reminded why it was best to keep quiet about innocent and harmless sounding pieces of information is that while one small piece of information may be useless to enemy intelligence, it can be of value when combined with other small bits of information. A big part in intelligence gathering is the collection of all available data and using it to try form an adequate intelligence picture of your enemies abilities and intent.
Granted the odds of an enemy intelligence asset overhearing the average civilians small talk and using that to get a better idea of what was going on was slim, but reminding everyone in a total war society of the need to follow basic operational security even at the lowest level isn't a bad idea in the least.
This propaganda picture gives you a basic idea of that practice. I also answered a similar question, here.
From The Truman Library's propaganda exhibition -
By the summer of 1942 more than 10 million people volunteered to help with this home front effort. And all citizens - but particularly those who worked in war-related industries - were constantly warned not to talk about their work or news they had heard from their relatives on the fighting fronts for fear that enemy sympathizers or spies would relay that vital information to the Axis powers.
Here's a Canadian propaganda piece from WW2 that illustrates why they were directed at the public.
Hi, you may be interested in a few examples...
not an incident, but an example of "spies among us"
