It is very different from continent to continent and country to country, but the not paid internships I know are for positions that prepare you for tough to get jobs or where the training the intern receives is far more valuable than the work the intern does. Some internships are closer to free schooling in a company with the option to get hired after 3 months. There are bad companies that use internship as cheap or unpaid labor, but that's the abuse of the system, it does not means it is always bad.