Benefits of using Nav Walker?

I would say mostly you see the usage of wp_nav_menu(), which does use wp_get_nav_menu_items(). You can use wp_nav_menu() with the default walker or a custom walker.

To be honest I can’t remember the time I did a menu manually, with wp_get_nav_menu_items() or something else, and I tend to be lazy, so this is definitely more work.

The Walker Class gives you a lot of possibilities as there is more information available, which definitely is big plus. So whether you should leverage a walker – or otherwise (I’d say) use wp_nav_menu() – is pretty much a question of what you need to achieve, for more complex menus I personally would always go with a walker. Last but not least, walkers are nicely reusable, which is pretty neat and another plus.