Three reasons to declare a font size in a page’s CSS:
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Not all browsers support all types of zoom (standard, pinch-to-zoom, and text-only zoom). Some mobile browsers still don’t support any type of zoom.
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Different use-cases call for different sizes. Long-form text should be bigger than text in user-interface controls.
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WCAG guidelines recommend sizing tap-targets to at least 44-by-44 CSS pixels, with the exception of inline links. Google recommends 48-by-48 CSS pixels, with no overlap across a 56-by-56 pixel range. Increasing my root font size makes non-inline links and buttons bigger.
Combining the first and second points, we see that the base font size on the Web is not a “one-size-fits-all” solution. It’s now a reference point for relative font sizes (e.g. em, rem, percents). Relative font sizes scale with the base font instead of “overriding” it.
My browser’s default size is perfect for a typical Fediverse microblog or information-dense discussion-forum, not for reading an article. My website’s stylesheet defines a font that is 9.375% larger than default, whatever that may be.