I chose to get a Personalausweis to not continue to have to use my passport as an identity card (as I had to as a Briton). I got it a month or so after my passport, to ensure that the two documents do not have the same expiry date. It wasn’t essential or compulsory to have one, but the use outstrips the financial outlay (ca. EUR 65 when I applied in Summer 2018).
How a Briton in Vienna chose the citizenship of his host country after the UK Referendum on EU Membership to negate the potential personal consequences of Brexit