The general's bulletin is the only reliable means to convey unit settings (and other related info) outside of the army structure. Why would you want to do this outside of the army structure?
1) In some realms, there's never any need to change them. You'll change marshals 4 times before doing the slightest tweak to your formations.
2) When there are multiple armies, a centralized location for info can be handy, to avoid differences between armies.
3) And perhaps most importantly, because adding people to armies requires active participation from a lord, and sometimes, you'd rather new nobles be able to simply jump right in with the armies and use the right settings right away, instead of waiting for their lords to assign them to the right army.
I'm sure people have thought of other reasons.
I have no idea in what hole you play in, but I've seen unit settings in general bulletins more than half the time. Perhaps for a simple 4th reason) Generals tend to have no frigging idea what else to put in the damn thing.
