When you study music in high school, college, music conservatory, you usually have to do ear training. Some of the exercises, like sight singing, are 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. But most people are busy and might be difficult to practice regularly. To get really good results, you should practice a little every day. If you do not have good options to get singing lessons near you, the GNU Solfege program/software could be of great help.
GNU Solfege tries to help out with this. With Solfege you can practice the more simple and mechanical exercises without the need to get others to help you.