It's a Latin American thing. Primarily for security, which is why it is most common in places that have had a recent history of internal conflict, and Medellin would be at or near the top of that list for the region. But also Bogota, Lima, Caracas, etc. In my experience, restrictions have actually loosened over the past decade as life in these cities has gotten back to normal. Before, it was common for anyone who was not a registered guest to not be allowed beyond the lobby, period. The other reason why visitors are often asked for ID, which is photocopied, is to make sure that the guest's companion is of legal age to consent to companionship.