Even the most fantastical setting needs a level of verisimilitude to prevent the suspension of disbelief. Mapping is an easy way to provide it because the nature of the map provides information on a wide range of things. Gravity ( water runs downhill ), basic thermodynamics ( climate, wind, etc. ), and social interactions ( populations, towns, and travel routes ) can all indicated indirectly by maps. These things give the players something they have direct experiences with and can relate to as a grounding for the fantasy aspects of the setting.