Amir E. Aharoni, 04/10/2014 15:09:
Is it a usual practice in the German Wikipedia?
Yes.
What is the reason for it?
GFDL
Is there a policy page about it?
Probably in the neighbourhood of https://de.wikipedia.org/wiki/Wikipedia:Importw%C3%BCnsche
Are there are Wikipedia projects that do this?
No, only German projects. The list of projects which make special use of import is at https://meta.wikimedia.org/wiki/Importer
Nemo