Theories at each of these levels have claimed to help us understand why war happens. However, there is no way to objectively say which level contributes the most to our understanding. Theories on each of these levels force us to think about how complex the causes of war are.
Looking at these theories, for example, we are forced to think about whether individuals are important in causing wars to occur. Would WWII have happened if Hitler had been killed in WWI? Realists would argue that it would because war comes about due to the fact that states need to pursue power as a way to protect themselves in an anarchic international order. Other theories, of course, would place a great deal of emphasis on the actions of leaders such as Hitler, Churchill, and Roosevelt.
We are also forced to look at whether/how the nature of the international order leads to war. We have to think about whether bipolar worlds are more stable than multipolar worlds or unipolar ones. Within the scope of an answer of this length, we cannot really explore which type of theory is most plausible. All we can do is to say that each type of theory complicates our thinking about war. Each type of theory makes us consider the roles of more kinds of factors in causing wars.