It goes without saying one requires international passport when travelling abroad, but what are the reasons for taking a national ID? In what situations may I need it?
Is that enough to take just printed copies of internal ID?
Situations when your national ID is useful abroad:
Of course in none of these cases it's a necessity, so you don't need to have with you.
I always take both my national identity card and my residence permit with me. They are a perfect replacement, when a hotel or any other non-official entity requires a proof of identity. Usually I start by giving my resident permit, then my national identity card and if that wouldn't work, I would consider giving my passport.
I would say that you need your passport to cross borders, but for any other use you could revert to your national identity card.
No it's not necessary, because not all countries issue national identify cards therefore not every traveller is able to obtain one.
Here in Australia a national ID card called the "Australia Card" was proposed in the mid 1980s. Most of us found the very idea Orwellian and abhorrent. The idea was so overwhelmingly unpopular it was totally withdrawn within about two years.