Why do you have both a book and a card??
I had a passport card, I regret getting it. When I got it they told me that almost everyone who gets one ends up getting a book later down the road. I still decided to get a card, and then I started this process and needed the book

The cards are pretty much useless, you can only use them to drive to Canada or Mexico, you can't even fly with them!
1) It was just an extra $20 to get a card when I got mine.
2) For an additional piece of government ID, especially since I do not even have a driver's license (can't drive). It's the only photo ID I have that's wallet-sized now.
3) For cases when I don't have my passport book with me (update, if it gets lost, PPR someday maybe, have had to send it in original for one or two other things before too).
I don't see how a relatively inexpensive additional piece of government issued ID is useless, particularly one that can still help you get back and forth across the border by land in a pinch.