Tomcat Ah, because I needed only the mesh so that I can place it directly on the Packed Scene with MeshInstance3D as root component.
Each building has the same two materials: Color Palette and Civ Color. So, for the whole game I only need a two material resource objects that I apply to every unwraped model. This makes it so the renderer finishes rendering in only two drawcalls (because all of the models use the same material). At least this is how Unity did it.
I never used .GLB but it seems to be that there is no easy way to add resource material to it. Also, it creates a packed scene that I can't edit by placing StaticBody3D and CollisionShape3D under it.
I didn't play with .GLB long enough and I am much more familiar with the .FBX or .OBJ formats in games.