The mail arrives, and you sort through it, trashing advertisements, setting aside bills, and deciding which of the catalogues you may actually look at, but a letter always gets opened first. Letters are scarcer than they used to be. These days even post offices send messages electronically. If you want to know something right away, that's the big advantage to it.
Speed is coupled with convenience.
There's no need to buy stamps or walk to the mailbox. A couple of key strokes and, presto, your typed message can be accessed by a computer on the other side of the globe. Conventional letters have been around almost as long as people knew how to read and write. They're addressed to a person or a group of people, but once the thoughts are written down they have a permanence that may carry them through the ages. It's possible for us to read books of letters written by historical figures like John Adams or Harry Truman or literary figures like Ernest Hemingway or E.B. White.