I'm going to try and answer this more from the Japanese perspective, since that's what I'm familiar with, but hopefully that can give you an idea of what might have driven the alliance between the three Axis powers, despite one of their major members being too far away for any kind of effective cooperation.

The question of whether to align with Germany was a rather contentious one in Japanese political circles. An alliance with Germany carried substantial costs, as it would almost certainly further sour already strained relations with the United States. Moreover, the Germans and the Japanese did not alway see eye to eye on matters of policy. Throughout the 1920s and 30s, Germany was a major supporter of Nationalist China, providing substantial economic and military aid, even as Japan expanded into Manchuria in 1931. When the Second Sino-Japanese War broke out in earnest, many of Chiang Kai-Shek's best divisions were German trained and equipped, with German advisors. It was only in the latter half of the 1930s that Germany began to turn more to Japan as its ally in the Far East, rather than China. The Anti-Comintern Pact of 1937 seemed to solidify this, aligning both Germany and Japan agains the Soviet Union, which both saw as an existential threat. Even so, the Molotov-Ribbentrop Pact of 1939 seemed to scupper much of the potential of a German-Japanese military alliance as it pushed the potential of military cooperation against the Soviet Union further away. This apparent betrayal of the core principles of the Anti-Comintern Pact pushed Japanese-German relations to their lowest point in the interwar period, espescially as the balance of power between Japanese and Soviet forces in the Far East swung against the Japanese, and the China War sucked in more and more of Japan's military resources.

So as we can see, there was substantial disagreement between Germany and Japan on questions of policy, and there was little coordination between them. The Germans were not exactly forthcoming with their own secret negotiations with the Soviets, and the Japanese effectively pursued their own policy regardless of what Germany was planning. While their interests aligned in some respects (more on this later), they also diverged in others. So, what changed between 1939 where Japan and Germany had seemingly split, and 1940 where Germany, Italy, and Japan signed the Tripartite Pact?

In many ways, it was the outbreak of the war in Europe, and the apparent German dominance of the Continent.

It's iimportant to remember that--throughout all of this--Japan was heavily involved with the Second Sino-Japanese War, which sucked in resources at an alarming rate, as the Japanese repeatedly failed to achieve a decisive victory that would force the Nationalist government to the negotiating table. The war in China was repeately souring US-Japanese relations, even as Japan sought to negotiate some kind of settlement with the US that would enable Japan to end the China War on its terms. An alliance with Germany (who seemed to be establishing itself as the new hegemon of Europe) was seen as a valuable deterrent to a US military response, or an added bargaining chip when it came to forcing the US to negotiate, or agree to Japanese terms.

If we look at the language of the Tripartite Pact, you will note that in addition to recognising each other's spheres of influence in Europe and East Asia (Articles 1 and 2), the Pact also includes a provision that the various signatories would come to each other's aid "...if one of the Contracting Powers is attacked by a Power at present not involved in the European War or in the Japanese-Chinese conflict." (Article 3) Furthermore, Aticle 5 states that, "Japan, Germany and Italy affirm that the above agreement affects in no way the political status existing at present between each of the three Contracting Powers and Soviet Russia." These provisions make the target of the Pact clear: the United States. The pact did not obligate Japan to aid Germany in its invasion of the Soviet Union or the war with Brritain, nor Germany to aid Japan in China, but only against a power which was currently uninvolved. From the Japanese perspective, the hope was that when faced with the prospect of a war in the Atlantic and Pacific, the US would be more willing to agree to some kind of negotiated settlement in East Asia, and allow Japan an escape from the impossible situation it faced with the China War and American pressure. Similarly, from a Italian or German perspective, the threat of Japanese action in the Far East would force Britain to further stretch its already strained resources to defend its colonial possessions in Asia, limiting Britain's ability to continue to prosecute the war.

Although the Pact did not allow for direct military cooperation, each nation saw a great deal of deterrent value agains their current or potential rivals in allying with the other.

To switch from the perspective of a historian to that of international relations, in terms of nations, there was a strong push for Germany, Italy, and Japan to ally. In many ways, all three were revisionist powers, seeking to fundamentally alter the international system as it existed at the time, defined by the dominance of the European colonial powers. In this, they had a great degree of common interests, espescially when it came to overcoming the status quo powers whou would seek to maintain the system as it currently stood. However, this is r/AskHistorians not r/AskIR, so I will refrain from wandering too far down this path.

I hope this helps to answer your question, and if you have any follow ups, I'll do my best to answer them.

Sources

• Edward J Drea, Japan's Imperial Army: Its Rise and Fall, 1853-1945 (2009).
• Stuart Goldman, Nomohan, 1939: The Red Army's Victory that Shaped World War II (2012).
• Eri Hotta, Japan 1941: Countdown to Infamy (2013).

Well that’s a good question. After all, why even have an alliance if there are no benefits to it. But let’s get one thing out of the way. This alliance with Japan was more an alliance based on coincidence. For example. Both the Germans/ Italians had a similar enemies as the Japanese. The Americans, British/ French, and Soviets are of course the most important. So obviously the Germans/ Italians and Japanese were going to form some kind of treaty. But again, this alliance was not really in depth and both powers often directly went against one another. For example, the Germans trained Chinese soldiers who were fighting the Japanese and the Japanese refused to sink any American ships sending supplies to the Soviets. So not exactly what you’d think of as allies.

But back to your question, what benefits. Well first off, I think of the United States and the dividing of their forces. I believe the US had 91 divisions in the field, out of that, approximately 25 or so of these were in pacific fighting the Japanese. This is similar for the Indian, and commonwealth troops who had to be diverted to fight the Japanese or European axis powers. I believe this is the most important thing of this alliance as it made the allies had to divide their forces and focus on multiple fronts. Those extra 20 or so divisions the US had in the pacific could have been used in the ETO very effectively ( of course they’re a lot of variables )

Another important thing is diplomatic backing. If one of these nations did someone, such as the Germans say annexing land, it would give the Germans legitimacy if another government backed them, for example, the Italians would do so when the Austrians joined Germany. I imagine this gave some credit in the international community to these nations.

But again, it was an alliance based off of common enemies and both nations actively went against each other at times. But I believe the most important thing from this alliance is the dividing of allied forces. If the US, UK and others were able to use their undivided armies and navies in one theater, the war for Germany/ Italy and japan would have ended much sooner.

As for the submarine and the Graf Zeppelin, I’m sure there are other historians who can give more information on that. It’s really interesting.