These are the best kind of letters, and the worst kind, but I don’t suppose people write them very often anymore. As Doctor P has pointed out, almost all written correspondence has been replaced by email.
Email love letters have their place, of course; but the speed and immediacy of email completely changes the whole pattern of the modern long-distance relationship. Especially when one person completely misreads the tone of a message, which always happens.
Email makes a handwritten love letter even more special, though, not least because it suggests that the writer concentrated on you for more than two minutes. Even if it says the same things that you tell each other every day, it’s just nice to see it written down (and let’s not forget the contractual nature of a signed document!).
Now, if you’ll excuse me, I am busy with my correspondence.