This is probably easier to follow if you have read Clausewitz, and understand the pressures pertaining to an on-foot army. E.g. the references to "foraging" would today probably be called "looting".

The central organisational challenge was: how to march 250,000 men on foot across a continent with no motor transport for supplies nor radio communications? It was more effective to spread out, but this required delegating authority down the hierarchy. Effectively the Grand Armee was a set of smaller micro-armies that allowed it to overcome the scaling challenge.