Entertainment value. I get the feeling that this Ii not one of those solo-levelling type manga where the this egoistic mc tries to monopolize everything and wants to be heroic saviour of the world alone. So the whole world and all the readers can tell him "what a great man this man is?". Nothing satisfies a man's ego quite like that. Hence the millions of fanatic fans of such genre.
This manga is more communal & less individualistic. It will be interesting how the author plans to take this further. Since the concept of fort & territory has already been established, I assume that human cities & towns within barracks & forts will come up eventually. (Like the necromancer manga)
And that let's me know that this manga isn't just about survival. Nascent human civilizations will eventually come up down the line if there are children & the elderly.
And moreover I think it was obvious when the author used soldiers as the main characters. Soldiers main duty is always to protect not to kill.
Why did they include the survivors and children with them?