System defaults can be ugly sometimes. Sometimes user might set their default font to something like Comic Sans or Papyrus accidentally or not and have no clue how to fix it. Users are really, really dumb. I wouldn't like my designed website to look like trash because the user wants to bring their ugly font, I want it to be consistently classic book sans-serif. Any time of the day I'd choose Segoe UI over Arial or god forbid Verdana. Hence that line and https://systemfontstack.com/ exist. The latter one is, by the way, always in my toolkit.
My website, my design. my fonts. Period.