Size and portability

The main advantage a command-line application has over a graphical one is the file size. Without the need to include a graphical library and all of its code, a command-line application can be significantly smaller. This not only allows for quicker downloads, but the ability to package the application on smaller forms of physical media.

Since all OSes will have a command-line interface by default, the potential issue of incompatibilities between a user's system and the graphical library in use will also be eliminated. Your application will also be able to run on machines which do not include a means of drawing graphical windows, such as servers, ensuring it can be run by as many users as possible.

Overall, the choice of interface for an application is largely down to what the application itself does and its intended audience. If you are building a simple application which you wish as many people as possible to use, then a graphical interface seems to be the way to go.

If you are creating a specific and very complicated program which is tailored toward people who are familiar enough with computers to not be intimidated by the command line, then the time and disk space savings of a command-line interface could be of real benefit.