There are a few advantages the Godot Shading Language has in comparison to GLSL:
1. fragment and vertex shaders are strictly separated but still use the same file
2. easy handling of shader types and render modes (e.g. the particles shader type and the light processor which would be a pain in the a** in GLSL)
3. ton of built in variables e.g. camera matrices, SCREEN_PIXEL_SIZE, etc.