One of the most difficult things to do is simulate game pressure when practicing shooting. Shooting around in the gym is a different feel from being in front of a sold out arena, and making a shot with the game on the line. Basketball drills like this are designed to help simulate some pressure that you will feel in a game.
Having to make 2,3, or 4 shots in a row with a teammate will put some pressure on the shooters and help them to prepare a little bit for game pressure. Knowing that you have to make a shot or you are going to let your partner down helps to simulate a little bit of pressure and will help the player become more mentally strong.