You are correct, Raino, about two of the uses but there is one more. Drywall/blueboard on a cieling on 24" centers will have a tendency to sag between the screw lines. Going to 16" centers will usually prevent this. Don't forget to stagger the ends of the sheets also so a crack that wants to start can only go 4'.
Joist material will have a crown and the truss builders are supposed to put all the crowns up. Good luck. Get a kijillion shingles so you can use these to fill in the spaces where the 1x3's don't hit the bottom of the joists and use a string line to insure that you are running the 1x3's level across the room.