The shell drill is going to work on half court man to man defense, and is going to really focus on the defenders responsibilities when the ball is moved, there are cutters, screening actions, etc. This particular shell drill is going to work on dealing with down screens and baseline penetration. By breaking down the defense into 4 on 4 and staying in the half court, you can really focus on making sure that your players understand their responsibilities. They are also able to get a lot of repetitions at what they should be doing.
Team basketball drills like this one are also great for working on defensive communication. Every time that the ball or a player moves, the defense should be communicating. This will not only help prevent defensive errors, but it will also intimidate the offense.