If the city is large enough, and you're selling a lot of different res, and you happen to have the granaries/warehouses far away from each other, the trader won't have the time to reach all of them in one month, resulting in impossibility to upgrade the route!
Being able to tell the trader which granaries/warehouses to visit would add a bit of logistic and a small challenge (build them close to one another or so). - as long as it doesn't interfere with the unsolved distribution issue, that is
#About Trading Routes' Upgrade
atomic condor
robust grotto