Some of the reasons you might want a GUI are:
- Some, or all, of the users are not comfortable with the command line interface
- It may be easier to use a GUI, due to the complexity of the application. Think about adobe illustratorAdobe Illustrator, the GUI is much simpler than writing postscript files or similar
- There are visualizations, graphs orand similar things that are better in a GUI than in ASCII
I suspect that the main thing that would motivate you, based on your description, is the first one. If you have some users that aren't comfortable with the command line interface, then you may want to provide a GUI.
One thing to consider, there are a few applications iI have used that basically build their GUI as a front end for the command line version, so there is a lot of shared code.