1. I'm guessing they found it easier to get a tight shot on the entire board that way. Either that or it's a design flaw and they just didn't feel like going to the expense of fixing the board.
2. I thought about that for a while and I'm guessing it's there in case Bill's button malfunctions. If you look at the floor, you'll see the player podium and the extraneous wall are joined by a mechanism that looks like scissors. If Bill's button didn't work, I'm guessing a stagehand could sneak onstage, pull that extra wall, and make the podium slide out without being seen on camera.
Both of those answers are blind guesses, it should be noted. I just felt chatty.