I always work from an outline, so I know all the of the broad events and some of the finer details before I begin writing the book.
Magic, like technology, is a tool.
If there are going to be people out there making war on other people, don't you think it's a good idea for some of those people to at least follow a code of ethics? Not 'honor' but something you can pin down and be sure of, something with the same rules for everybody.
These are my friends, my family. It would be hell on earth to spend the rest of my life leading them into situations where some of them are going to get killed... but it would be worse watching someone well-meaning but incompetent or untrained double those deaths.
I try to use all of my senses when describing a setting, and try to think of everything that would impact a character in any given scene.
It makes sense for people who are good at fighting to go out and do it-because if they're good at it, that means the fewest number of other people die.