When you study music on high school, college, music conservatory, you usually have to do ear training. Some of the exercises, like sight singing, is easy to do alone. But often you have to be at least two people, one making questions, the other answering.
This is ok, as long as both have time to do it. And if you sit in your room, practicing your instrument many hours a day, it can be nice to see other people 🙂 But my experience when I got my education, was that most people were very busy and that it was difficult to practise regularly. And to get really good results, you should practise a little almost every day. Not just a session before your next ear training lesson.
GNU Solfege tries to help out with this. With Solfege you can practise the more simple and mechanical exercises without the need to get others to help you. Just don’t forget that this program only touches a part of the subject.