Okay, the way it works is this:
Two things are generated by the number of formations (brigades or divisions) that a C-in-C directly controls: the number of tempo points(which are used to 'gain the tempo' and as PIPs like in DBx to do stuff with troops) and the army morale level, which is the number of formations multiplied by 2.5.
For army morale, every time a formation is broken, then a D6 is rolled and that number is taken from the army morale. When army morale falls below 0, the army as a whole is broken. But when at least 50% of the brigades in a division have been broken, the whole division is considered to be 'spent' (broken for all intents and purposes).
The other advantage is that an army organized in brigades will have more tempo points, so should be able to gain the initiative on turns where it really needs it.
But there are two big downsides. First, divisional generals are between 50% and 150% more efficient in turning PIPs into activity. Secondly, divisional generals, but not brigadiers, are represented on the table and are able to rally one shaken base for free per turn, which can be a key advantage.
I did a full review here which might help to explain any unfamiliar concepts: link
Regards
