The Digger Drill in Pokémon Brilliant Diamond and Shining Pearl gives you access to Secret Bases, a way to set up a little area in the Grand Underground where you can place specific statues. The Digger Drill is an item you’ll need to unlock by working on particular tasks in the game. Here’s where you can find the Digger Drill in Pokémon Brilliant Diamond and Shining Pearl.
You’ll need to speak with the old man who gave you the Explorer’s Kit in the first place, the one in Eterna City next to the Pokémon Center. Speak with him about the tasks he wants you to do and anything he assigns you to do. Eventually, he’ll give you a task to dig for fossils in the walls of the Grand Underground. When you return to him after completing this task, he’ll reward you with the Digger Drill.
The next task he gives you is to use the item on one of the walls to create a Secret Base, which is the entire reason to have the Digger Drill. You can use the item by having your character face a wall and interact with it. When you do, you’ll create a Secret Base where you can set down any of the statues you’ve found while digging for fossils in the Grand Underground.