Of course you can only see those errors when you have set WP_DEBUG to true in your wp-config.php file.
Why would you want to build a Custom Walker in the first place?
Well, for example if you want to add a description to the navigation menu, so that it reads both the menu item name and the description of it. After all description is one of the features you can add if you open the Screen Options menu on the Appearance > Menus page in the WordPress Dashboard!

