With three or more reasonably equal authorities, stability is better protected because if one begins to assemble greater power the others have reason and the combined power to join to put the first back into place. In republican government the legislative authority, necessarily, predominates. The legislative branch has become the trusted branch of government during the colonial period of Royal governors, and was seen as the body of the people, and hence having the best connection with and support from the people. It may even be necessary to guard against dangerous encroachments by still further precautions. In the compound republic of America, the power surrendered by the people, is first divided between two distinct governments, and then the portion allotted to each, subdivided among distinct and separate departments.
Some overlap is necessary for a true check.
As one scholar put it a century ago, the constitution provides a separation, not a disconnection. The fourth paragraph ranks with the first sentences of the Declaration of Independence, the Preamble to the Constitution, Lincoln's Gettysburg and 2nd Inaugural addresses and F. D. Roosevelt's four freedoms as foundation stones of American political culture, ideals, and structure.