After putting together the final module, its at the stage where things must be smooth and accurate to enable all four modules to meet (temporarily) when its time to be displayed so tracks align perfectly, allowing easy running of locos and rolling stock. So the orbital sander is used to take off any lip so that the 12mm ply top fits to the edge of the frame work.
Modules must be able to meet easily for good track alignment when the modules are setup.
When its track laying time tracks will perfectly join across the modules because the modules butt together with flush accurate edges.
Now just to do the other three..... oh and paint them...