Prepaid cards are good for teaching children about the limits and responsibilities of money, but they are also useful in very specific, practical ways. For example, the cards can be used online, a prime hunting ground for hackers and thieves. If you are nervous about your teenager using a card online, particularly if they are unsupervised, then the prepaid card could be the ideal solution. They are particularly useful because even if the card number is copied and re-used, only the money on the card can be used, and there won't be too much of a loss to worry about.
Prepaid cards also offer teenagers a real-world card using experience, which they might not have had with other forms of money. For example, most cards will allow the child to view their account online, so they can keep track of their spending. They may also be able to top the card up themselves using their phone.