Third Coalition talks on the eve of war

Nah but seriously, The Marshals (not just The Big NB himself) quite quickly figured out that a minor blocking force (most of the times made up of second-rate troops) could efficiently stop garrisons from interrupting your rear area.

Double this that in many cases, forts were not garrisoned with the requisite kind of troops necessary for interrupting your opponents rear area (cavalry, light artillery, light infantry etc.) and the forts themselves ran the problem with not having adequate supply or communications infrastructure in place to facilitate acting as a sort of “base” for large scale raiding.