There are multiple uses for this in most campaigns. World maps, local maps, city maps, player homes, and the current dungeon they are on. Being able to switch between them without the GM/DM being online to move what map they can see would be extremely useful. Specifically for me, I run a game where players are essentially fighting each other. In this game they have their own map where they can plan and defend. There is also a main map where all the players can visit and interact with each other. Allowing them to switch between their respective maps without me being on roll20 would be wonderful.