When a graphical element gets bigger, you can just scale it up normally, or redraw it. Scaling is obviously easier, but redrawing usually offers a more polished look as a bigger size affords bigger detail. This is seen across icons, sprites, everything.
It's also useful for fonts, although it's not something that is too prevalent on the web currently. The level of detail remains the same as all sizes, which means smaller text can look a little convoluted, and larger text can look a little chunky. In typographic history, size-specific design has been around for ages, but bringing it to the web would require many different font files - lots of kbs we don't want to use.
Variable fonts can have an opsz axis, allowing the design to change depending on the size it's used at. An elegant solution that requires less thought but with the same benefits.
https://pixelambacht.nl/2021/optical-size-hidden-superpower/