If you know where your at and know where you want to go, then you don't need a map.

But most people carry a map for those times where they aren't sure or don't know where they are or know where they are but don't know how to get where they want to go. And in emergencies, you'll get much faster response times if you can give accurate grid coordinates rather than directions like "by the little lake shaped like a shoe".

In my opinion, navigating without an accurate local map is a calorie burning losing proposition.