Well quite a lot actually.

Provided you accept one premiss, that there is clearly defined good and evil (well badness anyway) in the world, and that it exists in humans. This is the hurdle, but once convinced of this war has many benifits such as:

The noble death: Currently it is still impossible for human beings to live forever, this may change in the future, and if immortality is possible we will have to reavaluate a lot of our morals and ethics. Curently though we all die, and most get old and die slowly and decrepidly of debilitating illnesses. Putting the society to great cost and misery as we try and keep a dying organism alive for a bit longer so we can watch the person inside suffer just a bit more. Generally we loose most things we hold dear to ourselves on the final walk to death, bowel and bladder control, the ability to eat, think, make sensible conversation, walk, run, basically by the time we go our last moments are truly pathetic.

So war gives the opportunity to die with some dignity while trying to acheive the most noble of causes. The destruction of evil. Imagine going into battle, AK47 in hand, mowing down the harbigers of evil, and protecting your family and comrades. Then a stray bullet peirces your body armour and lodges close to your heart. It beats but a few times longer before its own pulses rupture the muscle itself and sends you to the other side. Your comrades come to your aid, praise you valiant heroism. You wish them well and exclaim a prophetic final sentence which will ring in their ears for years to come. And pass quickly, with dignity and a knowledge that your life was spent trying to acheive good.

Consumerism: In a world enonomy that relies upon continual growth, and thus constant consuming to survive. A war can be very benificial even necessary. It is possible their may be a limit to the amount one person can consume, and or own. This may be evidenced by the current flatening of the Japanese and American economies, they have reached consumption saturation point. So what better way to kick start consumerism than the mass destruction of basic needs and wants which just happens to be the by product of war. Bomb, blow up, shoot, disable, all those communication links and water supplies, computers and washing mahines. They will all need to be replaced and thus creating a huge demand, and kick starting economies all over the planet. The benefits flow to those countries not even directly involved in the war, as they can supply goods to the flattened countries. Provided credit is given of course.

Population growth: Lets face it there's a few to many people on this planet at the moment, well to many to sustain a western standard of living for everyone. So to kill off a few will at least postones the problem until better solutions can be found. It must be said though that modern warfare (the attempt to minimise calateral damage ) does diminsh the benifits in this area greatly.

Creativity: “It is not the quality of the artist, but the drama of his times that creates great art” (thats not a direct quote it just looks better that way). Some, dare I say most of the great works of art have come from the turmoil of struggle. The good versus evil, the struggle to survive, the sight of beauty in darkness. And what more darkness, turmoil, and conflict can you have but a war. It makes people think about what is truly important and reminds them of the mortality of being. It makes us think deeper and from this comes great heart felt works of art.

Togetherness, and charity: For some unknown reason in times of great turmoil and loss, and what better way to bring both than a war, people seem to give more to charity. And for no reason help there fellow man, often even those on the opposing side, without any expectation of reward or benefit to themselves. This also happens in times of natural dissaster.

The classic story is probably from the front lines in Europe during world war one where the opposing trenches, German on one side and English and French on the other side, joined in christmas carols together across the trenches and exchanged gifts. Even today you wouldn't see and Englishman giving a german they didn't know a christmas present, but because of war they did.

It also brings otherwise dissenfranchised communities together for a common cause, building friendships, love affairs and social groupings that can last long after the war itself, just look at the RSL (returned servicemans league).

Moving up the ladder quckly (pauper to prince):One of the classic romantic ideals is that one can be born into poverty and subservience and rise above it in a short period of time and become the beholder of power and wealth.Wars are a great catalist for this movement. Firstly a lot of the opposition, the people in the middle and at the top often get killed or shamed out of their positions, creating power vacuums that need to be filled quickly. Also war creates a world stage on which one can show their talents. Make yourself a hero and your one short step from leadership. The first gulf war did a lot for Colin Powell.

Advances in Technology: There are many things that we use and benifit from today in everyday life that came at the very least, more quickly, and some directly from war (nuclear energy, canned food). For in war times more resources are pushed toward research into things that are not just there to appeal to the consumer but have more to do with out doing the opposition and destroying or conserving human life. Also the ethical barrier that exists in peace time becomes much more flexible in war times, it is then possible to test almost untested products, on humans, such as new drugs and vaccines without the problem of whinging libertarians. the testees will probably be killed anyway so the ethical dilemma is greatly decreased.