Automated tools are good at catching objective, code-level issues like missing alt attributes or insufficient color contrast, but they cannot judge whether alt text is actually accurate, whether a focus order makes logical sense, or whether a screen reader announces a custom component’s state correctly. Manual testing fills that gap through direct keyboard-only navigation, real screen reader testing (typically NVDA and VoiceOver at minimum), and a trained tester’s judgment against the intent of each WCAG success criterion.
Most rigorous audits treat manual testing as the primary method and automated scanning as a fast first pass that flags likely problem areas.