The alternative is using a driver's license or a passport, both of which are also cards (or booklets), and government-supplied.
It is not like you'd be forced to get one, or carry it around. Finland has had ID cards as long as I can remember, and I never had to get one.
I had my passport stolen once, and identifying myself to the government to be able to get a new one was a bit of a pain in the ass, since I didn't have any other valid national ID (driver's license isn't considered one). The alternative way of authenticating involved maybe fifteen minutes of questions like What was your street address in 1987?