Earlier quoted context omitted.
I see it often in intranets, wikis, and PDF documentation. It's an additional context clue that you're leaving a closed website. The most egregious examples give you a separate click-through screen when leaving the website. Government web pages seem to do this the most. It would be nice if we could all use a standard icon or some other constant UI element--like a single underline for internal references and double un…
Yeah but why? What do people do with the information that a link is external? Do people think 'I'll follow this link - oh no wait a minute it's external I won't'?
Techies will know to just open it in a new tab, but most will have to remember to browse back to the site manually. It can be a disruptive process.