Defining page size before annotating ?

User guide states that paper size should be set before annotating.
https://librecad.readthedocs.io/en/latest/guides/dwg-setup.html#scale-and-dimensioning 

Why is this ?
I get the same result either way. Am I missing something ?

I think what the author wants to say is that at last when starting annotating and adding dimensions you should think about the intended printing scale, which in the logic of the author is determined by the available/intended paper size.
Maybe a bit misleading wording and should be expressed differently.