Familiarity

While developers may be very used to using the command line to interact with programs, the average user may not even know how to use it.

The majority of applications which people use come with some sort of graphical interface. It is only usually very niche and specific tools which will come with only a command-line interface.

If you want your applications to be accessible to as many people as possible, it is worth creating a graphical interface to make as many people as possible feel familiar when using them.