For those not in the know (or who didn't suffer through having to write CSS manually): font-display: swap changes the default behavior from "No text visible until font been loaded" to "Display fallback font until specified font is available", meaning you'll see Courier until Iosevka loaded properly, or Courier forever if the font file failed to load.
I'm guessing the website author added that after some "website performance tool" recommended it, as they make sweeping generalizations that don't always make sense in every context, and "always add font-display: swap so text is readable earlier" is now a dogmatic rule "web developers" (mechanical or wetware ones) add without thinking, so that precious performance number goes up.
I'm guessing the website author added that after some "website performance tool" recommended it, as they make sweeping generalizations that don't always make sense in every context, and "always add font-display: swap so text is readable earlier" is now a dogmatic rule "web developers" (mechanical or wetware ones) add without thinking, so that precious performance number goes up.