Whatever the purpose of the sites you work on, their success depends on visitors doing something. We want our visitors to sign up, or buy, or donate, or download, or apply, or post opinions, or pick up the phone and call us. One way or another if we are to judge our sites as being successful, they have to result in some kind of action on the reader’s part.
I once shared this when it was published, and thought highly of it. The post about it wasn’t very good, so I’m trashing it. But maybe this article is still relevant?