Glad you asked! Thinking of you cards are typically for instances where someone we care about needs a little extra thoughtfulness in their lives and as their loved ones, it's our responsibility to make that happen. If someone you know is going through a bit of a rough patch, sending them a thinking of you card is a simple -- but effective -- way to let them know they are not alone and ease some of the weight that comes with a difficult situation. It's a small gesture, but one that can go a long way.
On the other hand, you could also just send a thinking of you card to someone who has been on your mind... Literally someone you're thinking about. In this instance the card will act a lot like an I miss you card.