@neo14, to expand on what @sebavan said, the node material json is an artifact that is unique to Babylon.js. While it acts much like shader node graphs in other engines and can be converted to a glsl shader that is more portable, the node material format is an implementation unique to this engine. That means that the glTF specification does not support it so even if you do save to glTF, that node material will be ignored because there is no option to support it yet. The roadmap for glTF has always mentioned extensions for custom materials or shaders, but that is likely not coming soon.
What the glTF exporter is there for is for is more for updating properties that glTF does support, like PBR material settings, or for exporting an assembled scene. The latter, exporting an assembled scene is more tied to a playground where you can load several meshes and/or create meshes and PBR materials from scratch and then export to glTF. Since we have one inspector tool that both the playground and the sandbox share, the export option is there for both but finds more utility with the playground. I hope this make sense.