Creating of a map allows to plan potential game scenarios.
It also helps in visualising environmental concepts before final stage which is modeling.This is how it works: Concept artist prepares 3d visualisation of a room based on an outline selection on the map. Purposes of it are many:
Concept artist:
1. suggests colours, light and atmosphere of the level.
2. creates references for 3d artists.
3. helps to understand the sense of space, therefore again, along with the map provides reference for game designer responsible for the gameplay.