Viele WG's schreiben 'Hier is keine Zweck-WG' in der Beschreibung, wenn sie ein Zimmer vermieten wollen. Was bedeutet das eigentlich?
WG = Wohngemeinschaft, people sharing a flat.
Zweck means purpose, function.
So a Zweck-WG is a shared flat where people just share a flat to keep costs down, not because they really enjoy hanging out with each other. The people writing "hier is keine Zweck-WG" specifically look for Mitbewohner who want to interact and get to know each other.
The distinction is really only for pedants... but the former confirms that there is no purpose while the latter only refutes that there is a purpose.
Compare the structure with another example: "he does it for no money" vs "he does not do it for money".
Logically it's not something everyone grasps quickly, leading to the common misconception, for example, that atheism is "the belief that there is no god", instead of its true meaning "the lack of a belief in a god".