From the overview of U.S. Family-Based Immigration Policy (Feb. 9, 2018 R43145):
Family reunification has historically been a key principle underlying U.S. immigration policy. It is embodied in the Immigration and Nationality Act (INA), which specifies numerical limits for five family-based immigration categories, as well as a per-country limit on total family-based immigration. The five categories include immediate relatives (spouses, minor unmarried children, and parents) of U.S. citizens and four other family-based categories that vary according to individual characteristics such as the legal status of the petitioning U.S.-based relative, and the age, family relationship, and marital status of the prospective immigrant.
Those who favor expanding family-based immigration by increasing the annual numeric limits point to the visa queue of approved prospective immigrants who must wait years separated from their U.S.-based family members until they receive a visa. Others question whether the United States has an obligation to reconstitute families of immigrants beyond their nuclear families and favor reducing permanent immigration by eliminating certain family-based preference categories. Arguments favoring restricting certain categories of family-based immigration reiterate earlier recommendations made by congressionally mandated immigration reform commissions.