I've never understood the idea behind the "preview pane", all you do with email is read it, so what does a preview do other than show you the message? And how

I've never understood the idea behind the "preview pane", all you do with email is read it, so what does a preview do other than show you the message? And how is that any different from simply reading the email? Then again I've never been a power email user so maybe I just don't "get" it.