ehm, I would disagree. The key signature tells you what key the piece is in, period.
What the particular notes in that key do (their role, as you put it) is a property of that key.
What the particular notes in that key do (their role, as you put it) is a property of that key.
It's all about recognizing when to build tension and when to resolve tension.
Yes, the roles that the notes play is defined by the key, but if you don't mentally make the switch, you'll often perform things poorly because you don't anticipate where things are going correctly.
Or at least that's how it works for me, at any rate. Take anything I say with a grain of salt. I can't sight-read much at all.