Hold down the option key. Hover your mouse pointer over the green zoom button in the top-left corner of the window. A "move window to the left/right side of the screen" option appears.
It sort of does, but it's not as obvious as it should be. Mouse over the green zoom button in the top-left corner of a window, hold the option key on your keyboard, and then "move window left" and "move window right" options appear.