These tend to be very highly qualified interns, though. Landing an internship that pays this well requires a grueling interview process. And most applicants have advanced degrees (typically PhD's from the more well-known universities). It is generally a good way to enter the work force. In fact, without any other job experience to show for, this is often the only way to enter the work force. And at the end of the internship, most interns will be offered a full time position.
So, if you think of the type of internship you did in highschool, when you helped restock the shelves in your local supermarket, then you are thoroughly misunderstanding the scope of these positions. A more accurate view would be that this is an extended job interview. The candidate already passed all the other requirements (i.e. great resume, multiple phone screens, multiple in-house interviews,