Without being overly technical, consider that typographic characters sometimes have a little extra spacing ("leading") above the cap height and below the alphabetic baseline, resulting in the line height being larger than the font height...
For designers, this problem messes with the measurements provided by smart guides in Figma and other UI design tools. For developers, it means that they have to adjust their paddings, margins, gaps, and so on to account for all of this extra space.
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