As a good professional player one should play shots or replies towards the four corners. However, it is better or best sometimes to play a shot to the center or to the middle to change teh game plan and is mostly used in defense.
Actually speaking, hitting to the corners gives your opponent better angles for his shot placement. when your opponent plays a straight drop shot, if you reply with a net at the middle, you can cover both the angles easily (straight drop and cross court drop) and since, the shuttlecock is in the middle, your opponent’s shot angles are less dangerous.
As a general rule, play to the corners when your opponent has a slight advantage and When your opponent has a strong advantage, you should play to the middle to limit his shot angles.
The best example being, always play high serves should always be played to the middle. Because, if you play a high serve to a corner, your opponent will have plenty of time to move into the corner and get behind the shuttlecock and gives your opponent the best angles of attack.
The same logic applies to very high lifts and clears (for defensive purpose). If you play a very high lift or clear or a high serve play it to the middle.