The most elemental answer is that war consolidates power. Orwell uses the backdrop of constant war as a way for the government of Big Brother to strengthen its hold on power and social control. During a war, the organizing principle is law and order, structure and control. To engage in constant war, government exacts total force over its citizens. The historian Howard Zinn discusses such an idea in his work, A People's History of the United States:
"War is the health of the state," the radical writer Randolph Bourne said, in the midst of the First World War. Indeed, as the nations of Europe went to war in 1914, the governments flourished, patriotism bloomed, class struggle was stilled, and young men died in frightful numbers on the battlefields-often for a hundred yards of land, a line of trenches.
The implication is that the social discourse and action that might question governmental action and behavior is suspended during wartime, as patriotism becomes an elixir that all citizens are supposed to devour in mass quantities. Hence, constant war is a state of being where government is in complete control at all times.
There are, in my opinion, two reasons for there to be constant war in this book.
The first reason is so that there will be no luxuries available to the people. With constant war, the surplus resources of the society all go into making war materiel. This makes everyone relatively equal in the society -- there are no luxury goods that some people can have and others can't.
The second reason is to give the people an enemy on whom to focus their hatred. The people of the society have little chance for any emotions except hate and it is good to always have objects of hate so that people can vent these emotions. If they didn't have such outlets, they might become dissatisfied with the society and rebel.