Firstly, I dont condone war or violence, I think we are smart enough to resolve issues without it now.

But generally speaking, there are massive benefits if youre the winner in a conflict. New resources, new land, new labor force (assuming you dont wipe everyone out completely), reinforcement of your culture and ideaologies, and more. Theres a reason violece is inherit to human nature, it has a long history as a viable and successful strategy.