Not to mention that the security of your letter increases quite nicely when you use thick paper and seal it shut. Yes, with wax.
No, it's not super-secure, but it stops most casual snooping attempts. Taking off (and reattaching) a seal without breaking it is quite a bit of work if you choose the right wax.
Sure, if you're sending a letter. But as long as you maintain physical control over your paper it's far more secure than anything stored on a computer.
There's a reason Russia's FSB bought typewriters instead of a thought-to-be-secure computer system for the storage of its most sensitive secrets.