Without a skip link, a keyboard-only user has to tab through every navigation menu item on every single page before reaching the actual content, which becomes exhausting on sites with large menus. A skip link is typically visually hidden until it receives keyboard focus, at which point it becomes visible so sighted keyboard users can see and use it too.
WCAG success criterion 2.4.1 (Bypass Blocks) requires some mechanism to skip repeated content, and a skip link is the most common and best-understood way to satisfy it.