Useful for when you want to make sure the pages you're reading will still be around in some form in the future, and to easily get at additional context. (Just remember that it checks *every page you view* against the Wayback Machine, so turn it off when you're not using it!)

If you enable auto-save, be sure to add to the exclusions list so it doesn't waste time trying to archive your webmail or control panel.

There *is* a private mode where it doesn't do anything until you ask it to. But that leaves you with essentially the same features as a pair of bookmarklets to check or save the current page.