It may be useful to know if the current OpenGL implementation supports a particular OpenGL extension. For example you have coded a cool OpenGL 4.0 demo based on tessellation shaders. It would be nice to display a different scene (actually an error scene) if the demo runs on a system that is limited to OpenGL 3.0.
I didn’t checked all possible use cases of GLSL Hacker (a huge and actually impossible task) and some sequences of drawing instructions (render calls) can generate OpenGL errors. So if you suspect something dodgy because the rendering is not what you expect, you can check the OpenGL errors after some particular instructions.
Do you know that you can use data structures as uniform variables in GLSL?
OpenGL defines 3 modes for rendering a polygon: point (GL_POINT), line (GL_LINE) and solid (GL_FILL). The default polygon rendering mode is solid.