PCs will naturally associate with various people in their adventures. But they won't always know everything about them. Perhaps they'll become friends with a gregarious and helpful innkeeper who looks after them when they return after tough stints of monster fighting in the wilderness.
Imagine then if they find clues that lead to the conclusion that twenty years ago the innkeeper was the infamous necromancer who ravaged the countryside for miles all around, apparently now retired.
Or is he...??
Leia: Guys! Turn the ship around! You gotta see this!
Chewbacca: We are presently occupied with avoiding an exciting chase sequence with the inevitable Imperial fighters. You want us to turn around?
Leia: But Luke's a real party animal!
Lando: Luke? As in Starkiller? Number one bounty on the Imperial list?
Lando: Great, it was getting lonely me associating only with the number 2 and 3 most wanted people in the Galaxy.
C-3PO: Good news, everyone! I haven't downloaded the latest list and you're not number 4 now, Lando.
Lando: Uh, good?
Leia: Chewie, we totally need Luke, he can do awesome Force party tricks and stuff.
Chewbacca: Of course, there are potentially some even more important reasons for going back for Luke.
Leia: Oh yeah... We should totally fill a couple of kegs while we're there.