Why?  Diplo visibility.

Perhaps with a diplos escorting caravans one could see the size of a neighboring army encampment?  We see something similar to this by attaching diplos to armies and setting them within the diplo range of tourney squares.  This information can be helpful when sending the next round of clearing forces against opposition forces.

The caravan would be a sneaky method of accomplishing the same.  Of course, the caravan would have no defense except against another diplo force of the same type.

Allow these diplo-embedded-caravans to also see the size of any army within a city.  This would be similar to what is currently done when near NPCs or the previously mentioned occupation near a tourney square.  A harmless harvester could now see if a city contains "few", "pack", or "legion" of forces within, but no other specifics.

We all love paperwork in our inbox.  With sufficient sov around your city your serfs could let you know if you had a harvesting visitor.

I'd argue that this is more for the war savvy and it would be a research completed much later in the city building process.  Perhaps a level 20 foreign tower would be a requirement with a research time similar to diplo encampment within armies?

Just another one of my random thoughts that may well be subject to the public "poo poo".  Or is it an opportunity to suspect why someone in that aggressive BL and/or inactive-farming Elgean alliance is harvesting iron outside of your city?

Sleep well!