What is the primary reason to have three branches of government?

I know that in the U.S.A. we have three branches of government. The legislative, judicial, and executive. I think the main reason why we have them is because we cant just have one person deciding the laws, it needs to be approved by many people. Congress can even pass a law if the president veto's it!

- Middle schooler

explanation:

japan participated in world war i from 1914 to 1918 in an alliance with entente powers and played an important role in securing the sea lanes in the west pacific and indian oceans against the imperial german navy as the member of the allies.