It’s good to have both the Card and the passport.
The card is valid traveling by sea from Mexico, Canada, Bermuda, Bahamas, and the Caribbean.
Traveling by air or road, I think it’s only valid traveling from Mexico and Canada.
I mostly use my card for Canada.
In some countries, you have to show your local ID card, or passport a lot, in just about every store. It’s easier to just show the passport card.
Due to risk of theft, it’s customary in many Latin American countries to leave your passport in the safe and only carry a photocopy of the photo page and the visa entry stamp. I like having the official Card in my pocket too.
If you lose or have your passport stolen in a foreign country, wouldn’t it be nice to be able to show your card as proof of citizenship at the American consulate when you seek a new passport?