The AreaNav can be used for many useful things.
Finding trainers: Can't find anybody to battle? With the AreaNav you can find trainers to re-battle!
Quick access Flying: Instead of going into your party, tapping on the Pokemon with Fly and using Fly, just tap the AreaNav and select the area you want to Fly to. (If you didn't already know this, you can now Fly to routes in ORAS)
It's basically a quick access Town Map: A Town Map but much more convenient!
You can see what Pokemon you can find in an area: Similar to the DexNav and the Pokedex but combined! Just tap on the AreaNav, select your area and you will be able to see what Pokemon you have seen in an area.
Finding Secret Bases: Trying to find 1,000 Secret Base flags to get the Garchompite but don't know where to find the Secret Bases? You can easily find Secret Bases with the AreaNav! If there are secret bases in the area (Represented by little squares) and the little dots are white, you have not visited the base. If there are secret bases in the area and the little dots are gray/black you have visited the secret base. If there are no dots on the route/city that means that there are no secret bases there. It will also give a brief description on where the base is, it's not always easy to understand but it can be helpful.
Berries: Don't remember where you planted your berries? Or are just too lazy to see if they have grown yet? With the AreaNav you can easily find where you planted berries and if they are ready to be harvested yet!
Hope I helped!
(Now I sound like an infomercial guy)