I do that sometimes. I am not very good at expressing concern with my own words. I am always afraid I will say something stupid or wrong.Yet I do want them to know that i am thinking of them and care about what they may be going through.  a greeting card is a nice gesture. at least for me.

It's cheaper than buying them an actual gift.

I'm not as good at this as i ought to be. My wife usually does this for me, God Bless her. My wife and kids on the other hand really give me beautiful cards with alot of handwritten personal thoughts written on them. I usually get some nice cards from the rest of the family.