"flash gun"? Kind of an old term Do you mean an external speedlight? like the Nikon sb600 or the Canon 430ex? If so then there are many uses for the light.
- outdoors in bright sunlight shadows are harsh, a flash can be used to fill the shadows, not just on portraits but on any object.
- Macro photography almost always needs a light
- Any kind if set up studio style photos here you are shooting a small object, I'll set up lights on it