I don't understand the point of the rule sets and constant classes? TailwindCSS still obeys the same specificity rules of CSS so instead of `font-medium` on eve

I don't understand the point of the rule sets and constant classes? TailwindCSS still obeys the same specificity rules of CSS so instead of `font-medium` on every <a> tag under nav why not just put `font-medium` on the parent <nav> tag?