Line Art modifier is view dependent and generates lines only on the faces that are facing the camera. GP From Mesh is more like the Blender's convert tool, with the difference of being able to filter for sharp edges and intersections.

it is not necessary to install it if you do not need to disable the add-on. "SnC_Enable_Disable" is like a switch to turn on and off "Sketch N' Carve" without having to uninstall it. Its purpose is to release the keyboard shortcuts if they are needed for another function.
Once installed, you can find it under the menu: Object/SNC_Enable_Disable

Materials are applied automatically with random colors. Main purpose of the materials is to make geometry selections easier (using the "select" button below the materials panel in edit mode), particularly after some complex and overlapping boolean operations. However you still need to make UV maps, SketchNCarve is only a modeling tool so far.