In many countries (e.g. Czech Republic), the government requires that all tourists are registered with the appropriate authority: either through their accommodation or in person if staying at a friend's place.
What is the point of that rule? Bonus points for explaining the following sub-questions:
- If being registered is so important, why do authorities rarely check tourist registrations when the person is leaving the country?
- Why do some super-paranoid countries, such as the US, not bother with tourist registration?