it seemingly does nothing unless I already know where the rare is, it's on my screen, and I'm mousing over it to target it.
doesn't really help when it only finds things I've already located.....

Classic or Retail? If Classic, yes, that's all that's possible. If Retail, it can only detect things further out which have vignettes -- those stars on your minimap.

uh. stars? I don't recall seeing stars. is that for 100+ zones or something?
retail, yes, I know classic is way limited for everything, though I was hoping retail was...more helpful.
no wonder finding rares is so hard.

Pandaria and newer. Right around the time Blizzard added those, it also closed various loopholes that had previously worked for scanning for rares.

ah, naturally, they make a system that would make it better overall but dont apply it to older content and then break the stuff that does help with older content.