It is as basic as it sounds...
A Northmarker points the Interior map in the correct direction that correlates with the Exterior.
For Exteriors, North is always "up"
But Interiors have no North direction unless a NorthMarker is used. Therefore, the Interior map will not match the design of the Exterior unless a Marker is used.
If the Player is paying attention, it can be
very disorienting without one.
Use one. It is lazy not to include a NorthMarker for all Interiors.