Let me make a small correction. For permanent residents, they flog it in a different way. As long as you do not leave Thailand, nothing is required, you can stay forever.
But if you want to go out of Thailand for any reason, you have to purchase an 'exit permit' (dont know the official name). As they dont sell this at the airport and you may require to travel suddenly, most will buy a 12 month permit to leave & reenter multiple times. They do not sell this for longer duration, so the yearly pilgrimage to immigration office!