EDIT: It's worth noting that this is mostly a spoken thing, AIUI - most formal/semi-formal writing would be in Hochdetusch rather than a local dialect.
Historically, Germany used to be divided into countless small fiefdoms and each of them used to speak unique barely intelligible languages.
Hochdeutsch is in opposition to Niederdeutsch which Dutch and arguably English are a variety of.
The dialects are a whole other thing though.
Any literate German can read the NZZ easily, but they cannot have a colloquial conversation with an average person from Zürich, unless the latter switches to standard German (which is a foreign language for them, though one they have to learn from age 6).
I presume they also pick up a lot of standard German in the media: there's lots of German movies, and Germany has the biggest movie dubbing industry in the world, too. There's some Swiss German media, but not nearly as much as there's on offer in standard German.
As a native french speaker, no other language gives me that "why don't I understand what they say... oh, right, that's not my language!" feeling. Something with frequencies used, I suppose, but it always puzzles me.