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.
I started out as a substitute teacher and found it to be a terrific learning experience. It was a good way to discover that teaching was something I really wanted to pursue.
It's a place where you do some experimenting with class control, presentation, student engagement, etc. without having to worry so much about accomplishing a full curriculum. You also have the luxury of knowing that if something doesn't work one day, you can rework it and try it another day with a different group of kids. And, if you bomb completely, you don't have to worry about trying to getting the kids back the next day!
Simple! My cousin elected to become a substitue teacher (from having been a staffed teacher) for the following reasons:
It was important for her to be able to time with her huband, who maintained a job in which he often had full days off during the week.
Variety - one day you might be teaching math to 8th graders and the next day teaching english to 1st graders.
When you are part of a two-income family and may not necessarilly need a full time salary but want to work and can use the extra imcome to justify "luxury" items (and larger home, yearly vacation for the family, etc).
Negatives would be lesser income, no job security if you need to work as much as possible or are single and need benefits. There is also a bit of a lack of respect - from the full time teachers and the students with whom whom you generally don't have a relationship. And trust me, I remember being in grade school as well as HS and how we treated our subs...you think you can pretty much get away with anything!