Ugh. The feature has no purpose besides notifying people you're ignoring or haven't yet gotten around to. Not to mention that it's way too close to OKCupid's design where you can see visitors; I'd like to browse with some semblance of privacy.
It's actually also possible to implement this in email, although the loading of remote images is disabled by default in Gmail. Any email which contains a link can point to a web service which notifies someone, "hey, this page was accessed."
Facebook apparently doesn't let you turn this off.