What are the penalties for not doing so? I'm always amazed at the willingness of Europeans to follow rules like these, regardless of their impact on personal sovereignty. People in Finland were the most extreme example of this behavior that I've ever encountered. People would look like you murdered a child for jaywalking in Helsinki.
It's a nothing burger really. I have a card wallet for bank passes, transport cards and the id thingy. Not a single cop ever stopped me (I'm not in the usual suspect demographics, so that helps), so I didn't have to show it ever in 7 years. I imagine I would have a different attitude if cops did actually ask for it for no reason.
I however heard about things like riding a bicycle without lights and being fined 50 bucks for that, which triggers asking for an id, which in it's turn triggers a 100 euro fine on top. In the story I heard the second fine wasn't actually given.
Why carrying it is practical? What is it used for?
The only time I need my ID is during elections, but I can also show the one I have in our government produced app.
Older people in Poland do carry those, mostly out of some kind of habit and some kind of fear that police might need it. I can drive a car and get a speeding ticket, and all I need police to know is my ID number (it is not the identifier of my identity card), which I know by hard, it consists of my birth date plus 5 semi-random numbers). I don't need my ID, my driving license nor my insurance data - everything is located in police database based on my ID number (or my license plate).
Bad sides of carrying it is that you might loose it, and that is a PITA, because you need to block it right away (someone might take a loan with it, happens I kid you not) by calling your bank or similar service.
So I take it out from my wallet (which I don't carry also) only when I go to the airport.