Tabbed navigation can get small on smartphones, make it more difficult to 'touch' on touch screen devices. Side-nav fills the entire width of the screen, making it easy to select with your fingers.
The F-pattern describes the most common viewer eye-scanning patterns when it comes to blocks of content. The F refers to the reader first scanning a horizontal line across the top of the screen, as is common in cultures that read from left to right. Next, the viewer scans a vertical line down the left side of the screen, looking for keywords or points of interest in a paragraph or subsection titles. Then, when the reader finds something they like, they begin to read normally, forming horizontal lines.