using room will make it possible to design PCBs very fast.
If you have a hierachical schematic and for example 10 identical input circuits every ciceuit could reside in a room in the layout.
By designing one room and copy the design to the other room you save a lot of time.
At the moment this is done with hierarchical schematics and Replicate Layout python action plugin. There have been plans to integrate similar functionality directly to KiCad.