I was thinking about this subject since i started in web design and usability. Is left/right sidebar really needed or is it used just to fill blank space?
I'm talking about news/how-to websites.
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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.
At the moment more and more websites started dropping left/right sidebars because they want users to concentrate on content without distractions. Some websites only have the side bar at the top and the bottom of the post, to show related articles hoping that the user will stay on their website. Medium article "Fonts have feelings too" talks about it a little.
Rule of thumb, do not fill out the space just for the heck of it, it's better to leave it empty.
It really depends on what types of website you are developing. For news sites, now it's more popular to guide the reader through related news instead of providing a sidebar filled with category links.
This is a fairly new article on Webdesignerdepot that addresses the point of losing your sidebar to promote UX in the context of weblogs.
I can easily summarize the article in one line:
Elements normally found in a sidebar can also be placed inside the header.