Does anyone have examples? The dokumentation wasn't helpful in this regards.

Hello Matthias, 

Thank you for your question. 
The advantage of using categories is that you can set an assignee automatically but only if you set the category during the initial creation of a work package. Work package categories are restricted in that way that they are project-specific. We would therefore recommend using custom fields instead.

I hope this helps. 

Best regards
Samantha