Being the engine that drives each website, the navigation should be predictable, simple, consistent, and well-placed.
I used left-hand side navigation because it avoids cluttering a horizontal bar with too much text, and too many options. It also gives us more horizontal space for the navigation subcategories(Protiviti has 8 navigation tabs and some tabs consist of even 12 sub-navs and it has sub-navigations that have their own sub-navigation).
Another reason for using the LHS menu was that, when the navigation is sort of large, it's easier on the eyes to track it because we already used to read from left to right and by default, we expect that. Click on a category, and the menu expands to reveal sub-options… It also gives me more space to keep the page clean and highlight the main message executed on the homepage On the other hand, on the horizontal nav, in case the content is listed long, the user can simply scroll and click through.