The DexNav is one of the most important features as it can get you access to a variety of Pokémon. First, it can tell you what Pokémon are in the area you're in. Second, when you're sometimes in an area, you will see a Pokémon's tail coming out of the grass or water. Using the DexNav, you have the ability to use the search to find out data about this Pokémon. Using this, you can find out what ability, potential and if it has a rare move. You can then use the sneaking feature with the 3DS Circle Pad to approach.
If you have also seen a Pokémon with the DexNav, you then get the ability to search solely for that Pokémon to make things easier for you.
The search feature of the DexNav can be upgraded based upon how many Pokémon species you have found through the DexNav. As your DexNav level increases, you get a better chance of getting Hidden Abilities or rare moves. You will also get more details on the Pokémon when you scan it.
The DexNav has one more feature which allows you to register Pokémon in your Pokédex without battling them. This is done by interacting with them in the overworld in cities. In a ddition to that, you may see flocks, groups or schools of Pokémon in the overworld. This allows you to make sure you have the Pokémon in the Pokédex.