I guess one of the reasons that the authors of the Fighting Fantasy Gamebook series are exploring new concepts is because they need to experiment and change to continue to produce a viable product. The new idea that comes into this particular book is the concept of the skirmish, or the mass army battle. Unlike the other books, where you are generally on your own, in this one you are leading an army to go out and fight an army. Okay, there are other books where you are a commander (such as Starship Traveller) but this one experiments with the concept of the army and skirmish battles.
Basically a demon has raised an army and is threatening to destroy the freedom that exists within the land of Alansia (though since this is a medieval fantasy world, I suspect this concept of freedom really only extends to the freedom of the cities to rule themselves, though I have never found that the Fighting Fantasy books are either deep or philosophical in that regard). You, being the hero, and also as is indicated, being the one that succeeded in Deathtrap Dungeon (or Trail of Champions, it is not clear which) you are the obvious one to lead the army, though it is not as if you are tasked to do so, because you simply decide to do it yourself.
An interesting thing to note is that the idea of the army that is raised here is generally how armies worked before the development of the modern professional soldier. Armies were made up of ordinary people who were paid to pretty much give up their jobs and to go and fight a war. In some cases they were not even paid, or given the option, they were simply recruited. I have heard that Alexander the Great created the first standing army, which meant that he would have an army at his beck and call as opposed to having to recruit from the peasantry. However the problem with standing armies is that the farmers need to be able to produce excess food to be able to feed those who are not producing food. That means that if there are 50 farmers, and 50 non farmers, the farmers need to produce enough food to feed them and one other person.
However, in the world of Fighting Fantasy armies only exist while a war was in progress, and once the war was over, people would return to their normal lives. This was the case in Ancient Greece, though the Athenians tended to pride themselves on their democracy meaning that they would be more than willing to drop their ploughshares and go to fight. I guess that is also where the idea in the Psalms about beating swords into ploughshares come from, namely because once the war was over, and the farmer returned to his farm, he no longer needed his sword and would rework it back into a farming implement.