Many operating systems now have a "dark mode" preference. Some browsers already offer an option to transform webpages into a dark theme. The prefers-color-scheme media query is a way for an author to support their own dark theme, so they have full control over their own experience. The meta tag is needed so that a site can explicitly opt-in and say that they fully support a dark theme and that the browser should load a different UA sheet and not ever apply transformations.