Old Hollywood gimmick. Your lead needs a clumsy misfit to make him look more dominant and capable. In the book, Fredo's flaw is that he's just too dependent on family, reluctant to branch out. In the movie he's a total pushover and ultimately a traitor.

In the old westerns they used to have short doorways for the men to walk through and tall ones for the women, to make the men look bigger and the women look smaller. Juxtaposition. Weak, goofy Fredo compliments smooth, powerful Michael.