In my opinion, many teenagers have to work because their families need the money. Their parents might work shifts and not earn enough money to make ends meet. In these cases, it's good that teenagers can help their families by working in shops or restaurants. It's also true that a lot of young people want to buy things like clothes or music, and they don't want to ask their parents for the money. Working part-time is a great way for them to earn money for themselves.
However, I think that some teenagers work too much. They spend all their free time working, and this can be bad for them. For example, if they work late at night, they can be tired at school, and their schoolwork can suffer. Some people say that it's good for teenagers to learn about the world of work when they are still studying, but I'm not sure about this. A lot of the jobs that teenagers do are very simple and boring - serving food in a fast-food restaurant or stacking shelves in a supermarket, for example. The hours are long, and the wages are low. Teenagers often don't get much respect from their bosses or from the customers. I don't think that this kind of job is a good way to prepare for a career. In fact, I think that it can put you off working!
Of course, there are some good things about working as a teenager. You can learn how to manage your own money, and you can gain some useful experience of dealing with people. However, I believe that it's more important to concentrate on your studies at school and try to get a good education. With a good education, you can get a stable job that you enjoy and that is well paid.