Sidebars are considered "reserved spaces" for good reason. If you want a user to find a button / menu item quickly then the best place to put it is on one of the edges (this can be top, bottom, left, or right). This is because targets that are easiest to reach quickly with a mouse are the corners and edges of a screen / window.
If you don't have enough functionality to fill the left and right side bars then there is no reason to add content just to follow the convention. However, complex websites will have more than enough navigation requirements to fill at least one side bar.