The point was that generals shouldn't be giving orders to individuals, that's the marshals' job. Generals give orders to marshals. Generals handle strategy and marshals tactics, in a way, at least that's how it was intended.
For example, General Bob would order marshal A to take his army on the western front to clear the rogues, and marshal B to go conduct a takeover of a given region. When to leave, what regions to take, what formations to use, what takeover actions to favor, and such, would be determined by the marshals in due time.
Overall, this tends to give little responsibility to the general title, though, so the general is usually also a marshal. And honestly, while this is the idea, I've rarely seen it work all that well.
As for who assigns the marshals, this is a reflection of the feudal hierarchy. These aren't modern armies, with a centralized command. Troops are paid by a large number of individuals, with support from other individuals, under the command of many independent folk. The general has no de jure power over anyone, he's more of a public servant.
