Many students need the structure of a formal class setting, but some of them have learned how to study and work on their own. These are the students able to benefit from independent study. Many of them find a formal setting stifles their ability to proceed at their own pace, but others have a particular subject they want to explore. If their school system and a teacher are amenable, they are allowed to work on their own, and they are tested individually whenever they believe they have learned enough to move forward with their studies.
There is a downside to studying alone, and it is the lack of interaction with teachers and other students that often makes this type of class difficult for students of any age. Being able to share ideas with a group can help bolster learning, and it can help a student find future subjects to explore. Without the group, study can become a dull and boring job that simply must be completed.
One of the advantages of learning outside a formal setting is the ability to pace learning at an appropriate level. Working with others often means trying to keep up with them or holding on until they can catch up, and that alone can turn classes into exercises in frustration. Avoiding that can be important for personal development, and it can help a student excel at a class they might never have passed before.
Independent study is about work and discipline, and those unable to do either without constant supervision may never pass their course. Some schools allow for several semesters for each independent study class, but others have tighter deadlines so students do not get behind their class as they work on their own. Many students are able to take several independent study classes, and they can excel far beyond their grade level if they are willing to do the work.