Can maybe anybody explain why some settings can be found under Preferences and some under Document Properties?
I am for example referring to the snapping properties:

There is:
Menubar | Edit | Preferences | Behaviour | Snapping | ...
on the other hand:
Menubar | File | Document Properties | Snap ...

It's not clear to me why a snapping behaviour should be a document property?
Is there any reason behind it?

Someone made it document specific. It is saved in the svg file itself. Many people find it useful to be able to set and load different values for different projects they work on. You can also set them in your default template, to always get the one you prefer when you start out with a new document.