Symbolism, Imagery, Allegory
When Mr. B is trying to find Pamela's letters, he gets pretty personal:
I have searched every place above, and in your closet, for them, and cannot find them, so I will know where they are. Now, said he, it is my opinion they are about you; and I have never undressed a girl in my life; but I will not begin to strip my pretty Pamela; and I hope I shall not go far before I find them.
We could probably write a whole dissertation on the use of letters in Pamela, but we'll limit ourselves to two points. First, the letters are a symbol for privacy and bodily ownership. Specifically, they help Richardson make the radical point that even a servant has the right to control over her own body and her own thoughts. Whoa!
Of course, that doesn't mean that Pamela isn't willing to share them. It's just that she wants to share them on her own terms—just the same way she feels about her body.
Which leads us to our second point: the letters symbolize, well, letters—as in, literature. You may have noticed that seemingly everyone gets a crack at reading Pamela's letters, even though they are supposed to be private. There's a good reason for that: letters provide some good moral lessons (in Mr. B's case, one might be entitled "How not to be a sociopathic rapist"), along with lots of solid examples of Pamela's virtue, empathy, and kindness.
In short, they are instructive—which is what Pamela the book is supposed to be. In addition to just plain being the literary meat of the narrative, these letters—and, more specifically, the characters' uses of these letters—symbolize the impact that literature can have on the minds and morals of its readers.
And hey, at least Pamela didn't have to worry about the NSA.