Nobody forces you to do a factory reset after every update, but after a major update, it's a recommended practice on Android. If you don't have problems, you don't have to do it, if you get into trouble it's the first advice you get, because no one but you know what you've changed on your phone.
I've been using my ZF6 for over five months, making the first and last factory reset before the initial configuration in June. So far I have not encountered any problems that would force me into a factory reset.
It varies from user to user. For example, I use Nova launcher and original Google applications like Phone, Contacts, Calculator, Photos, someone else can do with built-in Asus clones of these applications including Asus launcher. Although these applications have some cool features, some functions, on the other hand, are missing and, above all, are not as well tuned.
