What is the purpose of immigration laws?

Answer 1

The simple answer is to control the movement of people into a country. Once you move into the question of why a nation wants to control the movement of people into it's country you open up a wide range of motives from control of communicable disease to flat out racial discrimination and all of these reasons play a part in the current debates.

I think the best place to find the official reasons for having Immigration laws is in U.S. is in the U.S. Code - specifically Title 8, Chapter 12, Sub-chapter II, Part II, Section 1182 "Inadmissible Aliens" where you will find concerns regarding the control of disease and crime. However the conditions described in this section only apply to immigrants as individuals not as racially defined groups and this is where they differ from quotas.

Quotas are a hangover from pre-1965 U.S. immigration laws that stated that Northern Europeans are a superior subspecies of the white race and that the white race as a whole, is superior to Jews and Asians. The purpose of having separate quotas for each source of immigration was to control the racial composition of the population based on this legal discrimination and when President Johnson signed the Immigration and Naturalization Act of 1965 to eliminate that discrimination he also eliminated the original purpose of the quota system.