Immigration checks on foreigners is a sovereign right of a state. Immigration checks prioritize national interest of Korea above all else, and it is meant to prevent admission of counterfeit passports, illegal aliens, and aliens who are barred from entering Korea.
All aliens intending to enter Korea must possess a valid passport and a valid visa issued by Korean Department of Justice. The expiration date on the passport and visa must be kept valid, with plenty of dates left, just incase the passport holder must renew them. Period of sojourn is 90 days, and the visa must be adjusted or renewed before expiration.