For example, there are three roommates and you know two of them well. You may not know the third person as well as the other two--or you just don't like them--yet you feel obligated to invite that person too so things won't get awkward.
Bob: . . . Amber, Emily, Roxy--
Cory: I thought you didn't like Roxy . . . ?
Bob: I don't. But they're roommates, so I just sent her a courtesy invite so that things wouldn't get awkward.