Unwritten Cards?

So in the past I've received many letters in which there is an unwritten card in the envelope and inside the card is the letter on other sheets of paper.  I've always been a bit flummoxed by this. For those of you who do it, is it because the card is so pretty you can't bring yourself to write on it? Or are you just giving me a card? The fact that the card surrounds the actual letter and there is no extra envelope included makes me think that it's not just that they are giving me a card.  I had a wonderful penpal who I've sadly lost touch with who did this all the time with cards that she had handmade herself. 

Any enlightenment? Have you experienced the same thing?

Well, here is one card that I received that way, and here at least it is clear why my friend didn't write on the inside.

 It is so colorful inside and out that it would probably be difficult to read what was written if you actually wrote on it. 

So after she sent it to me with letter enclosed, I sent it back after I had sewed some pages in and made it a sort of little book.

I love sewing paper together because it's so easy compared to actual sewing and because it makes me feel so accomplished. :-)

P.S. I don't mind it when people send cards this way (I'm always happy with mail!), I just don't quite understand why they do it!