Before your subs start roughing out the interior of the home, all "blocking" needs to be in place. These are smaller boards that are added in-between studs to give added stability and something additional to nail to.
This portion of blocking in the photo was added due to fire code. Any stairwell walls are required to have fire blocking. This in theory this slows the quickness of how fast a flame can move from floor 1 to floor 2.