Top navigation saves space

If we look at the common laptop screens and the portion that is taken by the navigation components, left navigation often will take 3 times more space than the top one. Adding to that some reserved header space for search or other elements like notifications or profile. 

Left navigation can easily take 25% of page space. Collapsing side navigation to save space will also not always work, as you cannot consistently fit long labels, and having icon-only navigation will strain user memory too much.