Description: The CSS outline property is an accessibility requirement, yet often abused by many web designers, why do they do it?
It provides visual feedback for links that have "focus" when navigating a web document using the TAB key (or equivalent). This is especially useful for folks who can't use a mouse or have a visual impairment. If you remove the outline you are making your site inaccessible for these people.
Defining focus to navigation elements is an accessibility requirement, it's clearly stated in the Web Content Accessibility Guidelines :
Usually "design vanity" or total ignorance of accessibility.