Are there any benefits of wars?
War indeed has its benefits, that's why wars happen. And there are, perhaps what we might call necessary wars, as the above examples about Nazi aggression and the Holocaust indicate. But they also tend to have a negative effect on civil liberties, on workers' rights, on civilian populations in war zones, and in a host of other ways. One post above mentions Israeli additions of formerly "forbidden" territory, and Chilean acquisition of Bolivian land without mentioning that these are clearly not good for the people who had these lands forcibly taken away. Ultimately, war is an exercise of state power. As with any act of power, someone benefits. Whether these benefits outweigh the human lives and treasure expended to achieve them is a much more complicated question.
Wars can be beneficial in all the ways already mentioned. World War II is probably one of the few instances in which both the victors and the losers benefitted from the war -- the victors by defeating tyrrany, and the losers by being freed from tyrannts. There are, unfortunately, very few wars like that one.
One of the benefits of war has been previously mentioned but the most famous example of it has not. World War II did more to lift the United States out of the depression than any economic policy decision did. The sudden demand for production that was then heavily or completely subsidized by government purchase of those products which was bound by contract created an incredibly powerful surge in production, hiring, etc.
In fact if you look at Orwell's vision of the future, a state of constant war was considered vital in maintaining a sort of status quo in an economic sense.
There are certainly few benefits--except to the victors, and that usually comes at a high cost in lives and money. As a previous post mentioned, the economy of some nations actually flourishes during war time, though the United States' own bad times during our recent excursions to Afghanistan and Iraq prove this is not always so.
There are some economic benefits to war (this is one argument mae to explain the US' semi-constant state of war for the last 20 years, but there is no way to prove this). Put simpley: War can provide jobs.
Socially, speaking there are some potential benefits to war. War can liberate oppressed peoples. War can remove oppressive regimes from power and/or tyrants.
All you have to think about to know whether war has benefits is the Holocaust. If WWII had not happened, it is very possible that even more Jews (and others) would have died in the Nazi camps. Even though saving them was not the point of the war, it was still one of the war's effects. When war saves people from horrors like this or from living under evil regimes, it clearly has had some benefit.
Wars also tend to focus research and development of new products and technologies, which can be adapted for civilian use when no longer needed specifically for military applications. Consider the advances in medical treatments that have come about as injuries from actions in the Middle East have led to increased awareness of brain injury treatment and developments in prosthetic limbs. It's a terrible price to pay, but there are benefits.
Although many downsides to war exist, there are also benefits. The US benefitted economically in both WWI and WW II. If you look at war as a way to keep the world safe, WWII did eliminate Hitler as a threat to world peace at that time. Other benefits may be the acquisition of land such as Israel in the 1973 war which enabled citizens to expand the boundaries and settle in previously forbidden territory. In South America, Chile has taken land which has closed off the Bolivian access to the ocean and the resulting ease of trade. Many times the benefit is natural resources like oil or water. You may look at the United States and its involvement in the Middle East as an example. If you look at the Arab world uprisings last year and this, Libya and Egypt are examples of gaining some freedoms in their civilian lives while Syria is still struggling. Therefore, yes, there are benefits to war especially if you discount the cost.