Grant was something rare in that or any war. He could learn from experience.
I think that everything you do helps you to write if you're a writer. Adversity and success both contribute largely to making you what you are. If you don't experience either one of those, you're being deprived of something.
But the same thing was true in the army. You slept in a barracks with all kinds of people of every nationality, every trade, every character and quality you can imagine, and that was a good experience.