The Pure Incense is one of the items you can have a Pokémon hold, but it has no effect during battle. Instead, this item makes it so you are less likely to run into wild Pokémon as long as the Pokémon holding it is the head of the party. This item is useful if you don’t want to waste money buying repels. Here is where you can find the Pure Incense in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get the Pure Incense, you first need to learn the hidden move Surf. You will automatically get this hidden move later in the game when you talk to Cynthia’s grandmother in the ruins. Unfortunately, you won’t be able to use it right away. To use Surf outside of battle, you will need to defeat the gym leader in Canalave City.
Now that you can use Surf outside of battle, head on over to Sandgem Town. Head south to Route 219 and use Surf to travel across the water and reach Route 221. When you make landfall on Route 221, head east and take the first northern path through the trees. Follow the trees to a dead end. The Pure Incense is hidden behind the trees out of view.