Some websites only have one column.
Some websites have multi-columns.
So why we need sidebar, or why we need multi-column page layout in web /print media ?
Is there any UX principles?
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Columns often equate to the density or volume of information to be conveyed. News sites use three or more columns to present more content near the top of the screen. But many marketing or portfolio sites only need a single column to convey the few key points they need to get across.
There is no rule; let your content guide your layout.