Substitute teaching is not always the most pleasant activity in the world, but it does have some advantages.
There are many drawbacks to being a substitute. Perhaps the worst of them is the feeling that you have no control over the students. Regular teachers maintain their control over students largely through their relationships with those students. Substitutes have no such relationships. Substitutes may not even know the students' names. This makes it very difficult to feel like you are in control of the class.
The biggest advantage of being a substitute is that it allows you to make some money on a flexible basis. This is ideal for people who are not in a position where full-time work is really practical or desireable. Substituting generally allows a person to make a pretty good daily wage without being tied down to a specific schedule. Of course, this also means you have no guaranteed income, but for a person who does not need the guaranteed income, it can be a good deal.