Why do some countries want to track the location of tourists after arrival?

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?

paperwork registration

Best Answer

There are few reasons for doing so :

for statistical reasons. At country level, it is important to manage the flows and understand how many people are coming in and where they are staying.
asking for the place where you stay is a way to ensure that you aren't coming to stay in the street and that you have an appropriate solution for your accommodation.
in the USA, they are asking for the reason of your stay and where you'll be staying. At least, this is what I saw in the ESTA. Isn't it a kind of registration (maybe not as formal as what you have seen elsewhere).
when you leave, they scan your passport so they know that you leave. What would be the need for checking registration again since you are leaving? As you leave, I guess the country authority doesn't care about where you stayed.