You must either own an ID card or a passport. This means having an ID card is optional. Does the German passport have a unique identifier as well? (never owned one)
The tax number otoh is assigned at birth.
There’s also another one which changes every time you move across financial administrative districts.
One motivation is that you want to travel to Israel and to some Arab country, which do not accept passports with stamps of the "enemy" in it.
I don't know if they verify the motivation in any way or whether it's enough to say so.
Seems like motivation enforcement is quite strict, as they do state that you can't even get two passports for the "enemy visa" reason when the visits are just planned, but you can't prove it yet (palne tickets etc.) you won't get it, and you'll have to go to the German embassy after you can prove it...
[1]https://www.bmi.bund.de/SharedDocs/faqs/DE/themen/moderne-ve...