The main benefit of Japan joining the Axis was to attack the British and French colonial possessions. The Japanese attacked the British held Hong Kong and French Indochina (now Vietnam, Cambodia, and Laos). British India was also kept at bay early on by the Japanese, who fought them extensively in Burma.

Additionally, the Japanese kept the Americans on edge, and the risk of conflict with Japan helped to discourage intervention in Europe until the attack on Pearl Harbor.

The alliance with Japan kept vast swathes of British, French, and American forces occupied. Reinforcements from colonial possessions did help the allies significantly, but tying them up early in the war helped result in the early string of Axis victories.

Had Japan not been part of the Axis powers, it is possible that colonial reinforcements would have resulted in Germany and Italy being defeated much sooner.

A great deal is made of America being brought into the war by Pearl Harbor but this is a pretty simplistic view of the era. America, by the end of 1941, was hardly neutral. It implemented trade policies that directly benefitted the Allies, at one point trading warships for military bases, and was selling large quantities of arms to the UK in particular. On the other hand, it implemented an embargo on Japan that aimed at depriving the Japanese of necessary war materiel.

It strikes me as unlikely that America would have sat out the war for much longer. Roosevelt was trying to find every legal way possible to assist the Allies and a concerted public opinion campaign certainly helped him dictate the general mood in the country. Yes, Pearl Harbor did cause America to cast off all pretensions of neutrality... but that's exactly what they were: pretensions.