On the whole, war is almost uniformly bad.
Materially, it often has advantages for the winning side, in that they can claim an awful lot from the losers to make good the costs and turn a tidy profit. (And let's face it, making a profit has probably been the greatest cause of war in history by a long, long margin). On the other hand, any advantages the winners get are taken from the losers, plus the losses of the war itself, so the total whole is a loss.
In terms of life, dead people don't come back, and as a rule the average person who dies in war will be no better or worse a human being than the survivors, so it's hard to see it as a good thing. Plus you've got deal with people who are injured and traumatised.
There are some situations - very few ones - where war may improve things. If a bunch of people are thoroughly nasty, murderous and destructive enough, the cost of war in life and material getting rid of them may be better than letting them continue.
Finally, just having to be ready to fight costs. Most Western countries spend about 2-5% of their GDP on the military, even without a shot fired in anger. That's a lot of money that could be spent on infrastructure, subsidising industrial production, or not taxed out of the population's pockets in the first place.