Option-clicking on a folder in the sidebar shouldn’t, theoretically, do anything useful: an Option-double-click inside a Finder window closes the window as the new item (another folder, or an application) opens. A plain click on a folder in the sidebar displays it in the current window, so why would you want to close it? Glad you asked!
I never tried Option-clicking on a folder in the Sidebar until I had set myself a challenge—find 20 Option-click tricks (yes, I should get a real life)—and was playing around.
Simply click a sidebar folder, and you’ll see its contents according to the settings of the current window. If you want to see the folder the way you last left it in a window of its own (the view, size, and position), Option-click on it in the Sidebar: the current window closes, and a new one opens with its personal settings.