My uncle sends me one of these letters every year. Basically it's just him boasting how many holidays he's been on and how wonderful his kids are and how rich he is (I'm reading between the lines on that one).
Basically only successful people do this. The losers like me don't bother.
I can imagine it now - "Dear so-and-so, I have spent the year wanking to porn and posting on Getbig. The End."